fix(php): update composer.json.dist (#21217)

`php/composer.json/dist` is 3 years out of date - changes have been made to `composer.json` which were not moved to `composer.json.dist` (which corresponds to the `composer.json` in the sub-repo [here](https://github.com/protocolbuffers/protobuf-php)).

The missed changes which are being included in this PR are:

 - https://github.com/protocolbuffers/protobuf/commit/3c8a3d27f7c035e977e2eb5226e4154f129a573b
 - https://github.com/protocolbuffers/protobuf/commit/d603b4199e42d18a6ed433f47bdf8e5042900588
 - https://github.com/protocolbuffers/protobuf/commit/75cc3a2af4693f14b27c7290d7f2148c77bce3d7

Closes #21217

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/21217 from bshaffer:fix-composer.json.dist 475e14bd7ad5b5b865bc958de447f453ed855f48
PiperOrigin-RevId: 749798569