[PHP] add support for field option "[deprecated=true]"

[cawolf]

What language does this apply to?
PHP (using proto3 with protoc-gen-php)

Describe the problem you are trying to solve.
As a developer of a service client, I would like to be able to identify deprecated message fields instantly in order to not use them anymore and replace them correctly.

Describe the solution you'd like
This could be implemented by adding a @deprecated annotation to the field accessors of the generated code, as already stated in the documentation.

Describe alternatives you've considered
Alternatively, triggering error_log() with a $message_type = E_USER_DEPRECATED would have a similar effect.

Additional context

[TeBoring]

LGTM, would you mind send us a PR?

[cawolf]

Sure, I opened #8223 - but the label check fails immediately, and I am not able to add the labels "php" and "release notes: yes" to it.