Restore buildSrc in jb-main. Changes in Gradle plugin

[igordmn]

Fixes https://youtrack.jetbrains.com/issue/CMP-9523/Copy-new-buildSrc-to-jb-main

Other PRs:
JetBrains/compose-multiplatform-core#2709
https://jetbrains.team/p/ui/reviews/101/timeline

This test fails after JetBrains/compose-multiplatform-core#2709 because of new constraints in ui-uikit

// ./gradlew dependencyInsight --configuration iosArm64CompileKlibraries --dependency org.jetbrains.compose.ui:ui
org.jetbrains.compose.ui:ui:1.9.3 FAILED
   Failures:
      - Could not resolve org.jetbrains.compose.ui:ui:{strictly 1.9.3}.
          - Cannot find a version of 'org.jetbrains.compose.ui:ui' that satisfies the version constraints:
          Dependency path: 'root project :' (iosArm64CompileKlibraries) --> 'org.jetbrains.compose.ui:ui:{strictly 1.9.3}'
          ...
          Constraint path: ... 'org.jetbrains.compose.foundation:foundation:9999.0.0-SNAPSHOT' ... --> 'org.jetbrains.compose.ui:ui-uikit:9999.0.0-SNAPSHOT' ... --> 'org.jetbrains.compose.ui:ui:9999.0.0-SNAPSHOT'
          ...

(it theoretically should be not resolved with any explicit dependency of "foundation" on "ui-something", but I couldn't reproduce it, might be a bug/peculiarity of Gradle)

Instead of downgrading a middle-level dependency, we downgrade high-level dependencies. We downgrade both ui/foundation because we will add a constraint for them in CMP-9710 Prevent possible version mismatch between ui and foundation

Release Notes

N/A

[igordmn]

Should be merged with JetBrains/compose-multiplatform-core#2709

[MatkovIvan]

That constraint doesn't solve runtime libraries (foundation/ui) <-> plugin mismatch