Migrate all Post Processing formatting to GAPIC generation / post processing

See https://github.com/googleapis/synthtool/issues/1415

We are currently running the NodeJS `prettier` plugin as part of `owlbot.py`, which for obvious reasons is not ideal.

We are already using the `symplify/coding-standard` package to format the code in this library (see [`Formatter`](https://github.com/googleapis/gapic-generator-php/blob/main/src/Utils/Formatter.php)), so I'm not sure what additional formatting is even happening using the `prettier` plugin . We may be using it only for the line-lengths. And if that's the case, we should either format the line-lengths with another plugin, or decide that this is not an important enough change and remove it from `owlbot` post-processing all together.