Remove unnecessary `to_i` for old sqlite3 adapter

`to_i` was added for SQLite3 adapter which did not handle number
but sqlite3 gem already supports it then `to_i` is unnecessary.

else condition is kept for adapters which return string,
i.e. mysql(not mysql2) and sqlserver.

Renamed `test_cache_does_not_wrap_string_results_in_arrays`
to `test_cache_does_not_wrap_results_in_arrays` to explain the
current behavior. most of adapters return integer, not only string.

* Refer these commits:

"future proofing the sqlite3 adapter code"
https://github.com/rails/rails/commit/beda2d43d6ac5c3435fc2fba0cbd108c20fe1c67

"Refactor calculation test to remove unneeded SQLite special case."
https://github.com/rails/rails/commit/47d568ed3fc701934ebe80b276f3d8bf6951c93f

"no need to to_i, sqlite does that for us"
https://github.com/rails/rails/commit/6cf44a1bd64ba10497742d70ad78fe68faa16e99

CI with the latest stable(GA) version of MariaDB 10.2

Fixes #30496.

when tested with MariaDB 10.2.8 or higher

Refer #30485

https://mariadb.com/kb/en/the-mariadb-library/alter-table/#drop-column-if-exists-col_name-cascaderestrict
> MariaDB starting with 10.2.8
> Dropping a column that is part of a multi-column UNIQUE constraint is not permitted.

- Travis CI is migrating Ubuntu version to Trusty
- MariaDB 10.2 is supported on Ubuntu Trusty

This is no longer used since 79a5ea9e.

since r59328

`def a() x = x = 1; end` warns since Ruby 2.5 (r59585)

Remove needless `silence_warnings`

Since ff30db13, warning is not show.

Clarify intentions around method redefinitions

Don't use remove_method or remove_possible_method just before a new
definition: at best the purpose is unclear, and at worst it creates a
race condition.

Instead, prefer redefine_method when practical, and
silence_redefinition_of_method otherwise.

Fix `can't modify frozen String` error in `XmlMini_JDOM`

Without this, `XmlMini_JDOM` raises an error as follwing:

```
RuntimeError: can't modify frozen String
    org/jruby/RubyString.java:2264:in `concat'
    activesupport/lib/active_support/xml_mini/jdom.rb:177:in `block in empty_content?'
    org/jruby/RubyRange.java:485:in `each'
    activesupport/lib/active_support/xml_mini/jdom.rb:174:in `empty_content?'
    activesupport/lib/active_support/xml_mini/jdom.rb:86:in `collapse'
    activesupport/lib/active_support/xml_mini/jdom.rb:66:in `merge_element!'
    activesupport/lib/active_support/xml_mini/jdom.rb:84:in `block in collapse'
    org/jruby/RubyRange.java:485:in `each'
    activesupport/lib/active_support/xml_mini/jdom.rb:82:in `collapse'
    activesupport/lib/active_support/xml_mini/jdom.rb:66:in `merge_element!'
    activesupport/lib/active_support/xml_mini/jdom.rb:51:in `parse'
    activesupport/lib/active_support/xml_mini.rb:101:in `parse'
```

We already have tests about `XmlMini_JDOM`. But it is not running in CI.
https://github.com/rails/rails/blob/master/activesupport/test/xml_mini/jdom_engine_test.rb

Ignore files already deleted on GCS file deletions

Relying on the GET request issued first to fetch the file we want to
delete is not enough to avoid this error. If the file is deleted after
our GET request but before the DELETE request we'll get a NotFound error
that after all means that the file is already deleted, so it can be
safely ignored.

In Action View Overview guide, remove reference to custom helpers [ci skip]

Support multiple versions arguments for `gem` method of Generators

Delegate :rindex, :slice, :rotate(missing) to 'records'

Unify the internal source control .keep file name

Run CodeClimate with the latest Rubocop

Update RDOC_MAIN.rdoc

[ci skip]

Add documentation about template partial caching

Motivation:
  - #29423 surfaced some confusion about how template caching works when
  using partials across files with different mime types. This doc
  attempts to explain how this can be accomplished.

Fix `can't modify frozen String` error in `DatabaseTasks`

Without this, `db:structure:dump` task raises an error as follwing:

```
can't modify frozen String
activerecord/lib/active_record/tasks/sqlite_database_tasks.rb:77:in `run_cmd_error'
activerecord/lib/active_record/tasks/sqlite_database_tasks.rb:72:in `run_cmd'
activerecord/lib/active_record/tasks/sqlite_database_tasks.rb:52:in `structure_dump'
activerecord/lib/active_record/tasks/database_tasks.rb:219:in `structure_dump'
activerecord/lib/active_record/railties/databases.rake:279:in `block (3 levels) in <main>'
railties/lib/rails/commands/rake/rake_command.rb:23:in `block in perform'
railties/lib/rails/commands/rake/rake_command.rb:20:in `perform'
railties/lib/rails/command.rb:48:in `invoke'
railties/lib/rails/commands.rb:18:in `<main>'
```

Update activejob doc [ci skip]

Update Action Mailer doc [ci skip]