Bump minimum PHP version

[colinodell]

Based on feedback in #625, the minimum PHP version will be bumped further to either 7.4 or 8.0 for the upcoming 2.0 release. This decision will be based on the following:

• Whether this library would significantly benefit from features only found in 8.0
• Whether the user base would be positively or negatively impacted by not having a maintained 7.x-compatible version
• Whether supporting 7.4 would be detrimental to the future development and maintenance of the 2.x branch

While I appreciate that there are strong arguments for/against dropping 7.4 support, I ask that any comments on this issue be respectful of the points above.

[markhalliwell]

PHP 8 introduces a LOT of nice syntax and features that helps clean up code and improve DX:

• Named arguments
• Constructor property promotion
• Match expression
• Nullsafe operator (best improvement IMO)
• Union types (second best)
• WeakMap class
• Stringable interface (can finally remove symfony/polyfill-php80)
• mixed type
• static return type

And so much more...

[colinodell]

Based on my in-progress work in #677 (and some un-pushed work), I've evaluated most of the major PHP 8.0 features and have come to the following conclusions:

• Type improvements (like union types, mixed, and static) would be awesome
• Syntactic sugar (like match expressions and the nullsafe operator) isn't overly helpful but we'd use it
• I have no use for named arguments or constructor property promotion
• We mostly use Stringable for return types so symfony/polyfill-php80 works just as well for our needs
• I have not yet found a use for weakmaps, though I'm still looking for one.

IMO PHP 7.4 offers more direct benefits to this library (like mb_str_split(), covariance, and contravariance) than PHP 8.0 does, but I'm continuing my efforts to determine whether the benefits of PHP 8's features justify adding another major upgrade hurdle to 70% of our users on PHP 7.x.

[colinodell]

After further reflection, I've decided that we'll continue to support 7.4+ in version 2.0 for the following reasons:

• We fully support 8.0 regardless
• There's nothing we need in 8.0 that isn't already being addressed with other viable solutions (like psalm, phpstan, and symfony/polyfill-php80)
• PHP 7.4 will receive security support for the next 16 months
• Version 3.0 of this library will likely be released in the next 12-16 months
• 70-75% of our users are not on 8.0 yet
• People have been waiting long enough for our 2.0 release :)

We'll therefore continue supporting 7.4 for now and drop support in our future 3.0 release.