Upgrade spotless-maven-plugin to latest version supporting git worktree checkouts

[groldan]

Upgrading spotless 2.20.0 to 2.20.21, which supports
git worktree checkouts as of diffplug/spotless#1119.

2.20.0 fails with an exception like the following for each module:

[INFO] --- spotless-maven-plugin:2.20.0:apply (default) @ geoserver---
[ERROR] /git/geoserver/main/.git/config: Not a directory
java.nio.file.FileSystemException: /home/groldan/git/geoserver/geoserver/master/.git/config: Not a directory
    at sun.nio.fs.UnixException.translateToIOException (UnixException.java:100)
    at sun.nio.fs.UnixException.rethrowAsIOException (UnixException.java:111)
    at sun.nio.fs.UnixException.rethrowAsIOException (UnixException.java:116)

Checklist

• I have read the contribution guidelines.
• I have sent a Contribution Licence Agreement (not required for small changes, e.g., fixing typos in documentation).
• First PR targets the main branch (backports managed later; ignore for branch specific issues).
• All the build checks are green (see automated QA checks).

For core and extension modules:

• New unit tests have been added covering the changes.
• Documentation has been updated (if change is visible to end users).
• The REST API docs have been updated (when changing configuration objects or the REST controllers).
• There is an issue in the GeoServer Jira (except for changes that do not affect administrators or end users in any way).
• Commit message(s) must be in the form [GEOS-XYZWV] Title of the Jira ticket.
• Bug fixes and small new features are presented as a single commit.
• Each commit has a single objective (if there are multiple commits, each has a separate JIRA ticket describing its goal).

[aaime]

Seems ok.