[CRINGE]: Replace master/slave references

[unclecheese]

Embarrassing and gross. Let's get this merged as soon as possible and set fire to all the previous commits. Thanks.

[NightJar]

Site won't load anymore, sorry
PHP Parse error: syntax error, unexpected token "(" in unclecheese/display-logic/src/Criterion.php on line 86

[chrispenny]

I think this would be a breaking change (protected is technically part of the API). Thankfully I was already planning a new major for Silverstripe 5, so I'll pick this up again as part of #152

[GuySartorelli]

Property names are part of the API as of PHP 8 as well, FYI. Another reason it should be done in a major.