dataconnect: gradleplugin now gets kotlin version from the main libs.versions.toml

[dconeybe]

The gradle plugin for Data Connect (located in firebase-dataconnect/gradleplugin) had its own, isolated libs.verisons.toml which duplicated the Kotlin version. This PR changes it to instead use the Kotlin version from the main libs.verisons.toml, in gradle/libs.verisons.toml.

Future PRs will eventually move all of the dependencies defined in firebase-dataconnect/gradleplugin/gradle/libs.versions.toml into gradle/libs.verisons.toml.

[github-actions]

📝 PRs merging into main branch

Our main branch should always be in a releasable state. If you are working on a larger change, or if you don't want this change to see the light of the day just yet, consider using a feature branch first, and only merge into the main branch when the code complete and ready to be released.

[github-actions]

Unit Test Results

  1 018 files  +     970    1 018 suites  +970   36m 48s ⏱️ + 35m 22s
  5 810 tests +  5 326    5 789 ✔️ +  5 305  21 💤 +21  0 ❌ ±0 
11 705 runs  +10 737  11 663 ✔️ +10 695  42 💤 +42  0 ❌ ±0 

Results for commit 5cf8f3a. ± Comparison against base commit ff3efc7.