Suggests to use a JSON structured log instead

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/54102

Extract internal route logic check out into #internal_route?
Add ? to #grack_route and #lfs_route predicate methods

As mentioned in
https://gitlab.com/gitlab-org/gitlab-ee/issues/9035#note_129093444,
Rails 5 switched ActionDispatch::Request so that it no longer inherits
Rack::Request directly. A middleware that uses Rack::Request to
read the environment may see stale request parameters if
another middleware modifies the environment via ActionDispatch::Request.
To be safe, we should be using ActionDispatch::Request everywhere.

Enables frozen string for the following:

* lib/gitlab/hook_data/**/*.rb
* lib/gitlab/i18n/**/*.rb
* lib/gitlab/import/**/*.rb
* lib/gitlab/import_export/**/*.rb
* lib/gitlab/kubernetes/**/*.rb
* lib/gitlab/legacy_github_import/**/*.rb
* lib/gitlab/manifest_import/**/*.rb
* lib/gitlab/metrics/**/*.rb
* lib/gitlab/middleware/**/*.rb

Partially addresses gitlab-org/gitlab-ce#47424.

So that we don't need to hold env after the request.
This makes it much harder to test, especially Rails session is
acting weirdly, so we need `dig('flash', 'flashes', 'alert')`
to dig the actual flash value.

after the request. This way, we could release the
project referred from the controller, which potentially
referred a repository which potentially allocated a lot of
memories.

Before this change, we could hold the last request data
and cannot release the memory. After this change, the
largest request data should be able to be collected from GC.

This might not impact the instances having heavy load,
as the last request should be changing all the time,
and GC won't kick in for each request anyway.

However it could still potentially allow us to free more
memories for each GC runs, because now we could free one
more request anyway.

This reverts commit 91075c82.

since `Rails.application.routes.url_helpers` creates a new anonymous module every time it's called

In !15082, we changed the behavior of the middleware to call
`Rails.application.routes.recognize_path` whenever a new route arrived.
However, this can be a CPU-intensive task because Rails needs to allocate
memory and compile 850+ different regular expressions, which are complicated
in GitLab.

As a short-term fix, we can do a lightweight string match before
we do the heavier comparison.

Closes #40185, gitlab-com/infrastructure#3240

- Also remove sign_out DELETE route from read-only whitelist routes

In GitLab EE, a GitLab instance can be read-only (e.g. when it's a Geo
secondary node). But in GitLab CE it also might be useful to have the
"read-only" idea around. So port it back to GitLab CE.

Also having the principle of read-only in GitLab CE would hopefully
lead to less errors introduced, doing write operations when there
aren't allowed for read-only calls.

Closes gitlab-org/gitlab-ce#37534.