DEP Limit PHP support for CMS 6

[GuySartorelli]

Issue

• Limit PHP support for CMS 6 to PHP 8.3 only silverstripe/.github#297

[tractorcow]

PHP 8.3 has a bunch of linting errors when using self:: in extensions. Do you want to address those in this same PR?

[GuySartorelli]

The linting errors aren't related to PHP 8.3. They have been addressed on the 7 branch in #867 - that just needs to be merged up for phplinting to be happy so that will be dealt with separately.

The trailing slash failures have also already been dealt with (in #872) and that just needs to be merged up.

I'll merge those up soon and rebase this PR on top of it, so we can see if there are any remaining CI failures to deal with.

[tractorcow]

Sounds good, go ahead and do those merges as you see fit.

[GuySartorelli]

Tried to push a merge up to the 7 branch and got an error "Changes must be made through a pull request". So I've raised #877

After that's merged I'll need to do another one merging 7 up to 8.

[tractorcow]

7 has been merged.

[GuySartorelli]

Thanks! I've created #878 merging 7 to 8.

[tractorcow]

I've left that PR approved; you can merge when ready friend.

[GuySartorelli]

Merged that, and rebased this on top. And it's green! 🎉