Run CI on PHP 8.4

[GrahamCampbell]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	N/A
Documentation	N/A
License	MIT

What's in this PR?

Runs CI on PHP 8.4.

Why?

To validate compatibility.

Example Usage

N/A

Checklist

• Updated CHANGELOG.md to describe BC breaks / deprecations | new feature | bugfix
• Documentation pull request created (if not simply a bugfix)

To Do

• If the PR is not complete but you want to discuss the approach, list what remains to be done here

[GrahamCampbell]

Build failures are not related, and should be addressed in a separate PR. Indeed, we need to turn off the PHP 7 incompatible changes that php-cs-fixer is suggesting.