Bump highest php version to 8.0 for 311 and master

[stronk7]

With core CI tests already passing for PHP 80 (see the php80 epic ), it's time to try it with codechecker.

So, bumping the highest runs of 311 and master from php74 to php80.

Let's see how it goes and if we need to perform any other change...

[stronk7]

Have created moodlehq/moodle-plugin-ci#94 because it seems that moodle-plugin-ci needs some love before plugins can run php80 tests.

[stronk7]

    Bump highest php version to 8.0 for 311 and master
    
    Only real required change  is to set the new
    max_input_vars=5000 requirement. Strictly speaking
    only needed for php8, but it doesn't hurt with
    older php versions and older moodle branches, so
    setting it up always.
    
    About the xmlrpc-beta extension, it's only needed
    for php8 runs and only if the plugin uses xmlrpc
    for something. If not, it can be omitted without
    problem, as far as it's not a moodle requirement
    but only a recommendation.
    
    Finally, note that we keep behat & phpunit init and run
    disabled, because there are some PHP 8 Warnings that
    need to be fixed (see MDL-71622) before enabling them back.
    
    For some reason GHA isn't able to detect those warnings and
    everything is passing here, so we continue covered.

[stronk7]

Self-merging to be back to green and have php 8 covered with GHA and (partially) Travis.