minor Gradle config fixes

[aSemy]

• remove old kotlin.assignment config
• skipMetadataVersionCheck=false Remove usage of "-Xskip-metadata-version-check" gradle/gradle#26810
• limit config-cache inputs Walking through project files causes CC to consider _all_ project files to be CC inputs, without warning, even though they are not used for Configuration gradle/gradle#28591
• Configuration.isVisible=true should be avoided, otherwise Gradle will expose configurations as default dependencies (the isVisible name, and Gradle documentation, sucks)
• remove old buildSrc files