Remove support for Java 11

[nedtwigg]

Some of our formatters can't compile on Java 11 anymore, so supporting Java 11 at all is getting harder. We'll support it a while longer, but its days are numbered...

• would make this issue trivial: Compile tests using Java 17 #1533

Running checklist of todo items to make this happen (please don't send PRs for these at this time, we aren't pulling the plug on Java 11 yet)

• remove CDT build complexity added in aab831c
• bump to latest JGit, which has worktree support
• can remove JGit system config workaround in 0c66cb0
• we'll need to bump minimum Gradle to 7.3, which lets us remove some compat stuff
  • Remove support for Java 11 #2375 (comment)

[nedtwigg]

Not sure why, but on Java 11, JGit was causing this to happen.

- Class `org.eclipse.jgit.util.FS`: external process started '/usr/bin/git --version'
  See https://docs.gradle.org/8.4/userguide/configuration_cache.html#config_cache:requirements:external_processes
- Class `org.eclipse.jgit.util.FS`: external process started '/usr/bin/git config --system --show-origin --list -z'
  See https://docs.gradle.org/8.4/userguide/configuration_cache.html#config_cache:requirements:external_processes

We don't have the problem on Java 17 or later. This hack fixes it

0c66cb0

But it would probably be good to remove it when we bump the minimum Java from 11 to 17.

[nedtwigg]

Another benefit to bumping to Java 17 is that we could use the latest JGit, which adds worktree support.

[nedtwigg]

It would also bump our required version of Gradle to 7.3, which would allow some minor simplifications here:

• spotless/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessPlugin.java

  Lines 30 to 31 in f22eda9

  	static final String VER_GRADLE_min = "6.1.1";
  	static final String VER_GRADLE_javaPluginExtension = "7.1";

• spotless/plugin-gradle/src/test/java/com/diffplug/gradle/spotless/GradleIntegrationHarness.java

  Lines 46 to 50 in f22eda9

  	JRE_11("5.0"), MINIMUM(SpotlessPlugin.VER_GRADLE_min), CUSTOM_STEPS(SpotlessPlugin.VER_GRADLE_minVersionForCustom),
  	// technically, this API exists in 6.5, but the flags for it change in 6.6, so we build to that
  	CONFIGURATION_CACHE("6.6"),
  	// https://docs.gradle.org/7.5/userguide/configuration_cache.html#config_cache:stable
  	STABLE_CONFIGURATION_CACHE("7.5");