Support slevomat/coding-standard 8.x + make PHP 8.1 default

[jonnyeom]

These are big changes. It is BC breaking and should be a new major release.

• Up selvomat/coding-standard to 8.x
• Up phpcs version to 3.7.x
• Remove php 7.x support.
• Upgrade fixtures with current methodologies of doctrine/coding-standard (for future maintainability)

[colinodell]

Feel free to remove the Generic.Formatting.MultipleStatementAlignment sniff too, if you'd like. I kind of hate it now :P Your call though!

[jonnyeom]

@colinodell
This change will make this fail on php 7.4 I realized.

What do you think of only supporting php 8+?

[colinodell]

I think that's fine since 7.4 is EOL and anyone who really needs 7.4 support can still use existing versions of this standard.

[jonnyeom]

Removed the Generic.Formatting.MultipleStatementAlignment. I dont mind but it does cause unnecessary changes sometimes

I think this is a good start.

At this point, we havent added much php 8 specific rules yet, I think we can add several rules but that can be discussed after an initial slevomat 8.x support release.

Please feel free to critique / suggest

[jonnyeom]

Perhaps it would be easier if we make a 4.0.x branch?