[FIXED JENKINS-34883] the legacy default update site can be different from the one specified by ID_DEFAULT

- [x] - Avoid String concatenation
- [x] - Switch to R/W lock instead of the specialized container with a significant overhead

Align /about: automation and not just CI server

* Test process cleanup can be suppressed changing BUILD_ID variable

Jenkins is an automation server, not just a CI one.

[FIX JENKINS-34857] Don't throw Exception when Jenkins.getInstance() yields null

Assign name to queue maintenance thread

[JENKINS-34833] - Installation Wizard: Allow altering the list of suggested plugins from update sites (#2348)

* [JENKINS-34833] - Installation Wizard: Allow altering the list of succested plugins from update sites

In our company we provide a custom package of Jenkins, which provides a customized set of plugins (open-source and closed-source ones). These plugins come from the custom update site.

We would like to provide custom suggestions to users of our Jenkins packages when they install Jenkins. For such purpose we would like to make the plugin suggestions list overridable by UpdateSite extensions in Jenkins.

* [JENKINS-34833] - Fix the JS Unit test (code from @kzantow)

* [JENKINS-34833] - Unit tests for SetupWizard

* [JENKINS-34833] - Address comment regarding the error propagation for the unknown version

* [JENKINS-34833] - SetupWizard::getPlatformPluginList() is not a part of the public API

* [JENKINS-34833] - Address comments from @orrc

* fix read of 'SystemProperties' values before init of the context

* fix constructor and test

* revert help-spec_fr.html

* revert help-spec_fr.html

* revert help-spec_fr.html

[JENKINS-34628] - Bundle "Font Awesome" and "Google Fints: Roboto" dependencies to prevent failures in the offline mode (#2318)

* [JENKINS-34628] Remove dependencies with external static resources

* [JENKINS-34628] License files were added

* [JENKINS-34628] Added references in the About page

* [JENKINS-32886] Pick up missing downloads

* [JENKINS-32886] Improve error handling.

[JENKINS-34755] Migrate to SystemProperties and restrict access to the engine

* [JENKINS-34681] Allow custom default plugin managers

* [JENKINS-34681] Convert remaining properties to SystemProperties.

* [JENKINS-34733] - Allow overriding Jenkins UpdateCenter by a custom implementation

* [JENKINS-34733] - Reduce the logging level for the custom update center selection

* [JENKINS-34733 and JENKINS-34674] - Use SystemProperties in the UpdateCenter

* [JENKINS-28071] generalise BuildCommand target from AbstractProject to Job

This allows e.g. WorkflowJob to build, fixing symptom [JENKINS-29826]

* fixup! [JENKINS-28071] generalise BuildCommand target from AbstractProject to Job

* fixup! [JENKINS-28071] generalise BuildCommand target from AbstractProject to Job

* [JENKINS-34745] - Prevent CheckUpdates PeriodicWork death in the case of the missing update site signature

* [JENKINS-34745] - Fix typo in the validator

* [JENKINS-34745] - Fix the formatting of the validation message (cc @lanwen)

* Fix NPE getting size of parameterDefinitions.size

* Protect get against NPE

* readResolve and adequate List management

* [JENKINS-34370] Simplify code

* Address comments

* Address comments

* Use NullPointerException

[JENKINS-34755] - Allow setting of system properties from context.xml in addition to setting from command line

* [JENKINS-19445, JENKINS-34213, JENKINS-34808] Bump remoting to 2.58.

Changes:
* [JENKINS-34213](https://issues.jenkins-ci.org/browse/JENKINS-34213) - Ensure that the unexporter cleans up whatever it can each sweep (https://github.com/jenkinsci/remoting/pull/81)
* [JENKINS-19445](https://issues.jenkins-ci.org/browse/JENKINS-19445) Force class load on UserRequest in order to prevent deadlock on windows nodes when using JNA and Subversion (https://github.com/jenkinsci/remoting/pull/81)
* [JENKINS-34808](https://issues.jenkins-ci.org/browse/JENKINS-34808) - Allow user to adjust socket timeout (https://github.com/jenkinsci/remoting/pull/68)

* [JENKINS-34121] - Upgrade remoting to 2.59

Conflicts:
	core/src/main/java/hudson/FilePath.java