Update dependency bazel_gazelle to v0.21.0 (v2)

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
bazel_gazelle	http_archive	minor	v0.20.0 -> v0.21.0

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Release Notes

bazelbuild/bazel-gazelle

v0.21.0

Compare Source

• # gazelle:exclude directives now accept patterns with * and ** wildcards. (thanks @​kalbasit)
• The -lang flag may now be used to select which language extensions should be run. By default, all extensions Gazelle was built with will run. (thanks @​robfig)
• Attribute matching now works on string slice attributes, not just strings. This is now used to match proto_library rules with one or more sources in common with the generated rule, even if the rule has been renamed. (thanks @​linzhp)
• Many bugs have been fixed. Thanks to everyone who helped out!

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