pin cs fixer to a version so we don't get unexpected updates

[bshaffer]

Our owlbot post processing uses a CS fixer to ensure the generated code is formatted correctly. The problem is that sometimes the underlying library updates in an unexpected way, and causes many PRs to be submitted for a minor (or unwanted) formatting change. This happened recently

We should pin to a specific version so that this doesn't happen unexpectedly.

[bshaffer]

I believe this happens in owlbot.py, where we use the npm prettier plugin, defined as plugin-php@^0.16:

google-cloud-php/DialogflowCx/owlbot.py

Lines 51 to 62 in 0e9d8cd

	subprocess.run([
	'npm',
	'exec',
	'--yes',
	'--package=@prettier/plugin-php@^0.16',
	'--',
	'prettier',
	'**/Client/*',
	'--write',
	'--parser=php',
	'--single-quote',
	'--print-width=120'])

If that's the case then it seems like we're already pinned, and I'm not sure why/how it happened. It's possible this is happening in gapic-generator-php also, and not here:

https://github.com/googleapis/gapic-generator-php/blob/277c7036edbcd245fc38cd3a97a761807af05188/composer.json#L34

This needs to be looked into.