Updated minimum supported PHP version for development to PHP 8

[danielbachhuber]

The minimum supported PHP version for development needs to be updated to PHP 8.

At the minimum, this impacts:

• https://github.com/Automattic/jetpack/blob/master/docs/development-environment.md
• https://github.com/Automattic/jetpack/blob/master/tools/check-development-environment.sh

Discussion in p1655132549395029-slack-CDLH4C1UZ

[jeherve]

Related: #22046

[anomiex]

Copying a relevant comment from Slack:

Re PHP 7.4, I don't think we're intentionally incompatible. The problem is normally in locked composer deps. When Renovate does its lock file maintenance it uses PHP 7.4 and locks to appropriate versions. But then every once in a while someone does something like #24334 that accidentally updates symfony and such to versions that require 8.0.

P.S. Yesterday #24707 happened, so things should be back on 7.4-compatible versions again. For now.