Improve Gradle-related Kotlin code

[MaciejG604]

After migrating the gradle scripts to Kotlin there are some issues that need to be resolved:

• Find a way to use version catalog aliases in helpers' code in buildSrc/
  (one way to do it would be to put the aliases into extra properties of the project in buildSrc)
• Unify plugins' notations in libraries and plugins in libs.version.toml:
  • the versions shared with the [plugins] sections should be extracted to [versions] section to avoid discrepancies
  • or maybe the [plugins] section can be removed completely in favor of the newly-added [libraries]?
  • extra care should be taken to retain @pins on certain plugins, as done in [plugins] currently (so that they're NOT auto-updated by gradle versionCatalogUpdate)
  • a comment would be useful why plugins in [libraries] are needed in the first place and/or a reference to a blogpost describing this technique
• Remove code that's marked as Deprecated from CheckerFrameworkConfigurator::applySubtypingChecker, then revert kotlinOptions.allWarningsAsErrors = true in buildSrc/build.gradle.kts
• Refactor IntellijVersionHelper so that there are no unsafe typecasts: preferably extract a data class to keep the IntelliJ versions retrieved from properties file, to avoid using string keys
• Switch all classes in buildSrc to Kotlin
• Remove all remaining support for Groovy from Gradle code (mostly Spotless)
• Check if it's possible to keep build.gradle.kts files formatted with Spotless (and not just files in buildSrc/) — see kotlinGradle { ... }
• Fix spotless for Kotlin so that it launches automatically with ./gradlew build
• Fix Gradle warning in :buildSrc:compileKotlin task: 'compileJava' task (current target is 11) and 'compileKotlin' task (current target is 1.8) jvm target compatibility should be set to the same Java version
• Check if there is any built-in way in Kotlin to use static identifiers for projects: something like api(project(branchLayout.api)). Probably no point in defining them explicitly if they aren't available OOTB => NO
• Check if it's possible to replace BuildUtils.foo(project) calls with just foo() (probably an extension method on Project would be needed) => YES
• Rewrite the latter half of UpdateEapBuildNumber#checkForEapWithBuildNumberHigherThan to use Kotlin regex API
• Consider un-hardcoding names of IntellijVersions properties by using .name: https://kotlinlang.org/docs/reflection.html#property-references

[PawelLipski]

(@MaciejG604 checkboxes FTW ☝🏻 )

[PawelLipski]

@MaciejG604 pls tick off once you start work on the given point. This will prevent doubling the efforts in case someone else would like to pick a subtask.

[MaciejG604]

I don't know if un-hardcoding names of IntellijVersions properties can be done while still keeping all the type guarantees. It is so becuase the class stores not only Strings (nullable and not-nullable) but also a List of Strings.
It was not hardcoded before using a Map<String, Any>, but the types could not be checked by the compiler.

[PawelLipski]

Ouch, sorry... I pointed to the wrong class. I meant unhardcoding keys in the dict used in com.virtuslab.gitmachete.buildsrc.IntellijVersionHelper#resolveIntelliJVersions ‼️

[MaciejG604]

Also related but can't link as there's a limit of 10 PRs linked:
#1035, #1038