Update caveats for google-cloud-sdk

[commitay]

After making all changes to the cask:

• brew cask audit --download {{cask_file}} is error-free.
• brew cask style --fix {{cask_file}} reports no offenses.
• The commit message includes the cask’s name and version.

Removed fish completion instructions from caveats. Closes #30847.

Fish completions have been removed from the unversioned url download.

While they are included in a versioned alternative I don't see how to make this a versioned cask without users having to update their profiles with each new version or using postflight to create symlinks somewhere so as to have a static path to the completions.

source '/usr/local/Caskroom/google-cloud-sdk/latest/google-cloud-sdk/completion.bash.inc'