Bump to Gradle 7.2, polish build and upgrade plugins/libraries

[simonbasle]

This PR migrates to Gradle 7.2, and upgrades most plugins (ensuring compatibility with Gradle 7) and libraries.

As a result, some adjustments had to be made both to the build and to replace deprecated / removed APIs.

The only exception is Byte Buddy in reactor-tools, for which a migration path isn't simple and will be treated separately in a following PR.

• Upgrade Gradle to 7.2
• Extract more library versions in root project's ext rather than constants in strings (eg. dependencies of reactor-tools)
• Upgrade most plugins to their latest version to ensure best compatibility with Gradle 7
• Upgrade most libraries to ensure we're not lagging behind (paving the way in some cases for a JDK upgrade)
• Polish the build
  • align a bit better with minor main changes
  • use actual Java version in messages rather than "9+"
  • don't require signing if publishing to MockRepository (in addition to MavenLocal exception)
• Fix the codebase to address warnings and errors:
  • fix usages of Duration with awaitility
  • fix tests in reactor-test where AssertJ message slightly changed
  • use JavaPluginConvention instead of JavaPluginExtension in buildSrc
  • fix some Gradle deprecation warnings (eg. removed feature preview, mainClass)
  • fix xxxRuntime configurations => xxxRuntimeOnly
• Fix subtle changes/regressions in the build result / improve detection of such cases:
  • fix exclude of reactor-tools shadowJar (and added a test)
  • catch cases where ArchUnit tests are not really run because classes couldn't be discovered

Here is a non-exhaustive list of upgrades for the most significant plugins and libraries:

• kotlin from 1.3.72 to 1.5.31
• shadow from 4.0.2 to 7.0.0 (along with attempt at simplified configuration)
• test-sets from 3.0.0 to 4.0.0
• bnd from 5.0.1 to 5.3.0
• changed the jsr305Version name to findbugsVersion and set it to 3.0.1 (replicating a change in main)
• JUnit from 5.6.0 to 5.8.0
• AssertJ from 3.19.0 to 3.21.0 (included some minor breaking and behavior changes)
• javaObjectLayout from 0.9 to 0.16
• awaitility from 3.1.12 to 4.1.0 (use of JDK 8 Duration)

[OlegDokuka]

LGTM

[simonbasle]

rebased on top of latest 3.3.x and squashed into a single commit. I need to quickly verify the forward-merge locally before merging because I suspect it won't be trivial.

[simonbasle]

Forward-merge needed a bit of adaptation for newly introduced ArchUnit test.

[reactorbot]

@simonbasle this PR seems to have been merged on a maintenance branch, please ensure the change is merge-forwarded to intermediate maintenance branches and up to main 🙇