Factor the PHP install used across CI workflows into a reusable composite action at .github/actions/setup-php.

The action takes php-version and driver (major 1 or 2) as inputs, resolves the latest stable release of the mongodb PHP extension for that major via the GitHub API (mongodb/mongo-php-driver releases), then chains shivammathur/cache-extensions + actions/cache + shivammathur/setup-php.

Goals:

• Stop recompiling ext-mongodb on every CI run (actions/cache step was missing in build-ci.yml, build-ci-atlas.yml, static-analysis.yml).
• Always pick up the latest stable release of ext-mongodb automatically, without hardcoding version numbers or bumping env vars.
• Remove duplication of setup blocks across build-ci.yml, build-ci-atlas.yml, static-analysis.yml and coding-standards.yml.