Prior to this commit, the TestGroup.CI enum constant was only used in a
single test method in spring-core. In order to enable that test, the
`testGroups` JVM system property was configured for the
Publication-master CI build plan; however, the `testGroups` system
property is not set when executing local builds. Consequently, there
has been a Gradle cache miss for every `test` task when building
something locally that's already been built on the CI server.
This commit addresses this issue by removing the `TestGroup.CI` enum
constant. The `-PtestGroups=ci` command line configuration for the
Publication-master CI build plan has also been removed on the CI server.
Closes gh-23918
01fb35bd2d