Update PHP support information

[mikini]

• elaborate on support states of upstream project
• don't call out EOL'ed 7.4 as default (use more vague terms about oldest actively supported branch)
• add 8.2 in example 'list php' invocation as is current reality
• use capitalization of states as PHP project's table (support -> Support)

Update support table:

• 7.4: Active support -> EOL
• 8.0: Active support -> Security Support
• 8.1: Active support -> Active Support
• 8.2: added

Refer also to support conversation about delayed 8.0 migration and postponed 7.4 sunset.

Note that text's assumption about the default PHP might not be true (the auto-migration to only security supported 8.0 seems to contradict the text, if rationale behind migration and default policy does not differ).

[nichtmax]

Hey @mikini, thanks! These are all changes that we rolled out with U7.15 a few moments ago: https://github.com/Uberspace/manual/pull/461

Can you check if you're happy with https://github.com/Uberspace/manual/pull/461 and update your PR accordingly? ☺️

[SalocinHB]

obsoleted by #461 and subsequent releases.