Add php8.6 to the test matrix

[jorbin]

As PHP 8.6 is still in pre-release, failures should be allowed and we should only run this with one DB for the time being.

See: ​https://github.com/WordPress/wpdev-docker-images/pull/223

PHP 8.6 is due out at the end of 2026. ​Beta 1 was released on August 13th.

This adds a PHP 8.6 job to the PHPUnit testing matrix, but configures jobs using that version to be allowed to fail.

Trac ticket: Core-65904.

## Use of AI Tools
None

I'm not sure that we need to be testing with every DB provider for the time being. What would you think of excluding some of the combinations from the matrix?

I've mostly reverted my original changes in favor of creating a new job solely for PHP 8.6. This makes it much easier to manage the number of combinations and the values passed to the various inputs for the reusable workflow without needing to add a bunch of conditional checks.

We could pare it back further if we'd like. I chose to go with one multisite and one single site job on both the latest LTS version of MySQL and MariaDB. It adds 4, but I think that's reasonable to get feedback while running the latest versions of both projects that WordPress currently recommends to users.

Looks good to land. We can adjust from there.

[desrosj]

In 63480:

Build/Test Tools: Run PHPUnit tests against PHP 8.6.

PHP 8.6 has entered the beta/RC phase of it’s release cycle and is due out at the end of 2026.

To help contributors prepare, the PHPUnit strategy matrix now includes this version. Since the test suite does not pass with PHP 8.6 yet, these new jobs are allowed to fail until the potential compatibility issues can be addressed.

Props desrosj, lancewillett, jorbin.
Fixes #65904.