Remove --ignore-platform-reqs from PHP 8.0 builds

Update PHPUnit to 9.3

$ composer update phpunit/phpunit nikic/php-parser --with-dependencies

Remove workaround in PDO\SQLSrv\Connection::quote()

Do not suppress warnings in PHP_CodeSniffer reports

The polyfill would cause a PSR1.Files.SideEffects.FoundWithSymbols warning.

Ignore code coverage of private constructors of static classes

Fix badges

Trigger Github workflow for merges

A merge is viewed by Github as a push to the branch where the merge
occurs, see https://github.community/t/trigger-workflow-only-on-pull-request-merge/17359/2
This means we should have coverage reports for merge commits from
Github, which should result in saner numbers.

Fetch parent commits

CI platforms do not run their jobs on the commit you push. Instead, they
run it on a merge commit so that both your changes and the latest
version of the target branch are taken into account.
Before uploading the coverage report, CodeCov's bash uploader attempts
to find the original commit hash, and succeeds on Travis and AppVeyor,
but not on Github, because the method it uses does not work unless the
parent commits are fetched too, which results in Github posting to
CodeCov with a different hash than Travis or AppVeyor.
Setting the fetch depth fixes that issue.

Additional deprecation note for PrimaryReplicaConnection::query()