Remove gemnasium-gradle-plugin and DS_EXPERIMENTAL_GRADLE_BUILTIN_PARSER env var from gemnasium-maven

Proposal

After we've Set DS_EXPERIMENTAL_GRADLE_BUILTIN_PARSER to tr... (#437277 - closed) • Philip Cunningham • 17.0 • On track and enough time has passed without major incidents, we should remove the gemnasium-gradle-plugin and DS_EXPERIMENTAL_GRADLE_BUILTIN_PARSER environment variable from gemnasium-maven

Implementation Plan

• Remove gradle-dependencies.json from mvnplugin.go
• Remove gemnasium-gradle-plugin references from the gemnasium-maven Dockerfile
• Remove gradle plugin installer from the config/install.sh script
• Remove DS_EXPERIMENTAL_GRADLE_BUILTIN_PARSER environment variable added by Add initial support for DS_EXPERIMENTAL_GRADLE_... (#437268 - closed) • Philip Cunningham • 16.9 • On track.
• Remove docs for DS_EXPERIMENTAL_GRADLE_BUILTIN_PARSER Remove Experimental Feature Flag from Documenta... (!149875 - merged) • Philip Cunningham • 17.0

/cc @thiagocsf @johncrowley