Update gersemi to v0.21.0

[PatTheMav]

Description

Updates gersemi CMake formatter to version v0.21.0.

Motivation and Context

Regular updates to the most recent release of formatters ensure that possible changes introduced by their updates are limited in scope.

How Has This Been Tested?

Tested on separate custom Homebrew tap.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.