Fix dry release Gradle config resolution

[marandaneto]

💡 Motivation and Context

make dryRelease was failing during Gradle configuration because PosthogBuildConfig loaded gradle.properties from a fragile relative path. After fixing that, posthog-server still failed under dependency locking because its Dokka Javadoc plugin entries were missing from the lock file.

💚 How did you test it?

• Ran ./gradlew :posthog:publishToMavenLocal
• Ran ./gradlew :posthog-server:publishToMavenLocal --write-locks
• Ran make dryRelease

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file
• Added the "release" label to the PR to indicate we're publishing new versions for the affected packages