[JENKINS-55048] Add support for plugin Java requirement metadata

Adds Polish translations

[JENKINS-55449] Add tool to update "@since TODO" entries

using the proposal from @jglick in https://github.com/jenkinsci/jenkins/pull/3016/files#r246146054

Provide more information about where/how the `Minimum-Java-Version` is coming from the plugin's metadata

as proposed by @jglick

This way at least we know the backend methods should not be broken without
noticing.

Given this field is present in maven-hpi-plugin, update-center, etc.
I think it does not hurt to keep the 'Minimum-Java-Version' name across
various usages to help make it clear.

Suggest ExtensionList.lookupSingleton

[JENKINS-29150] - Fix the CronTabTest.testTimezone test

Adds Polish translations

Update to latest version to support M2E

Simpler check for local file modifications

See https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/jenkinsci-users/FjzMrudsfb0/2oWXs9mZDAAJ
where we see Eclipse import fails.

See https://github.com/mojohaus/build-helper-maven-plugin/issues/43
And https://github.com/mojohaus/build-helper-maven-plugin/commit/f6d1fea90ccf7d9ef03e4587ac440d645f4c9898

translation-tool.pl supports backslash

[JENKINS-47458] update trilead-ssh2 version

Update to latest parent pom jenkins:1.51