Perform a complete PHPCompatibility 8.1 execution against codebase

This task is one of the final ones to perform when everything else in the epic has been already applied.

It's about to run the PHPCompatibility standard (that we usually also run as part of our local_codechecker) against the whole codebase.

Prerequisites:

1. The epic must be almost finished (to avoid detecting problems that correspond to other issues).
2. PHPCompatibility must support the php versions being checked.
3. Determine the range of PHP versions supported for the branch (7.4-8.1). See MDL-71747 for 4.1 reqs.

To perform the run against master, just:

1. Clone https://github.com/PHPCompatibility/PHPCompatibility locally.
2. Ensure you're in the "develop" branch (the "master" branch contains the latest release, quite old now 2019).
3. run composer install
4. Run:

   export DIRROOT=/path/to/your/moodle ; vendor/bin/phpcs \

   --standard=PHPCompatibility \

   --runtime-set testVersion 7.3-8.0 \

   --extensions=php \

   --ignore=$DIRROOT/vendor/,$DIRROOT/local/ \

   --cache=/tmp/cache -p $DIRROOT

5. Look for detected stuff, discard false positives (it can be useful to look to the results of the previous PHP 8.0 Epic - MDL-73586) to copy any decision here.