GH Actions: test against different versions of all CS dependencies

[jrfnl]

As things were, when testing against "dev" versions, only PHPCS "dev" was used, while WPCS now has more dependencies, i.e. PHPCSUtils and PHPCSExtra.

This commit updates the workflows to take that into account better.
It also improves the readability of the workflows by using multi-line command in a few places.

Notes:

• This renames the phpcs_version matrix variable to dependencies in all workflows to make it clear this is not just about PHPCS itself, but about all dependencies.
• The CS check for the own codebase is always run against "dev" versions of all dependencies as an early detection system for upstream issues.
• The "Ruleset" checks are run against low/stable/dev.
• The fixer conflict check is run against dev only, as conflicts detected cannot be fixed anymore in already released versions.
• The "quick check" now only runs against low/stable.
• The "test" run run against both low/stable by default and now has some extra builds against "dev" versions.
  • These extra builds could be set to "continue on error", but I've chosen not to do so as if any issues are detected, they should be fixed asap as they will impact WPCS sooner rather than later.
  • Code coverage will now be run against low/high PHP with low/stable dependencies and no longer against dev.
• Also note that, while the "quick test"/"test" runs shouldn't need PHPCSExtra, as that is a dependency used only in the ruleset, not in our sniff code, I've still included PHPCSExtra in the upgrade/downgrade block for consistency.

These changes do mean that the "required build"/branch protection will need to be updated (again) before the PR can be merged.
I will do so once the PR has been approved.

[dingo-d]

Left just one question. It's more about consistency across the workflow files.