feat!: add owlbot automated updates

[bshaffer]

Adds compliance with AIP-213

• depends on cl/521835772
• depends on chore: drop support for PHP 7.3 #56
• fixes https://github.com/googleapis/common-protos-php/issues/50
• unblocks chore: add phpstan static analysis google-cloud-php#5931

Note: I've removed all the alias files because we are going to tag a new major version with the new protobuf updates. And without deleting these in the PR, the owlbot update was causing the tests to fail.

see #55 for what this repo looks like after removing everything in src and metadata and regenerating with owlbot.

[bshaffer]

FYI @chingor13 @SurferJeffAtGoogle I am doing some unusual things with version and the copy paths here. I am not sure how else to do this, but if there's a better way please let me know!