GH Actions: update for php-coveralls 2.6.0

[jrfnl]

PHP-Coveralls 2.6.0 has just been released and includes a fix for the last known PHP 8.x issue.

This means that it should now be safe to install php-coveralls on PHP 8.x and upload from there, which means we now only need the work-around for the PHP version when on PHP < 5.5 (as Coveralls v1 does not work with GH Actions).

Ref:

• https://github.com/php-coveralls/php-coveralls/releases/tag/v2.6.0

[jrfnl]

Note: the build will fail until PR #304 has been merged. After that, this PR should be rebased to get a passing build.

[sirbrillig]

304 has been merged.

[jrfnl]

Rebased!

[sirbrillig]

Thanks!