Simplify Checkstyle configuration

[artembilan]

• Move checkstyle-conventions.gradle content to the build.gradle since it is short enough
• Remove checkstyle-header.txt and its handling in favor of Spring Javaformat out-of-the box logic
• Upgrade to Gradle 8.5
• Remove suppression for Javadoc* checks since it is handled by Spring Javaformat
• Leave only MissingJavadocMethodCheck exclusion from SpringChecks since all others really make sense for our code quality

[artembilan]

It looks like Spring Javaformat has to be upgraded to the latest Checkstyle to make them all compatible with whatever we expect.
I mean that even Gradle 8.5 doesn't help 🤷

[artembilan]

It looks like burrunan/gradle-cache-action scans console output for error and fails the build.
Unlike whatever we have with the jf gradle for SNAPSHOT build and when we run locally.
The ignoreFailures = true indeed has an effect, but with burrunan/gradle-cache-action for PRs it fails gathering all the errors in the end for final failure 🤷 :

BUILD SUCCESSFUL in 7m 28s
515 actionable tasks: 394 executed, 121 from cache
cacheid: 1737
cacheid: 1738
Cache Size: ~0 MB (5774 B)
Cache Size: ~0 MB (4638 B)
Error: [Task :spring-aggregator-function:checkstyleMain] [JavadocPackage] Missing package-info.java file.
Error: [Task :spring-aggregator-function:checkstyleMain] [SpringMethodVisibility] Method 'messageStore' in private class should not be public.

[onobc]

LGTM thanks @artembilan