Bump PHP requirement to 8.1

[lucaswerkmeister]

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While this has been contentious in other libraries in the past, my position is that this should not be considered a semver breaking change, because:

• PHP < 8.1 is no longer supported by PHP.
• Composer takes the PHP requirements into account when selecting a library version. If someone is still on PHP 7.4, Composer won’t install a newer version of this library, so nothing will break for them.
• The last bump (ef70bb5) was also in a patch release.

[lucaswerkmeister]

Bumped the target version from 8.0 to 8.1 following @manicki’s suggestion.

[lucaswerkmeister]

While this has been contentious in other libraries in the past, my position is that this should not be considered a semver breaking change…

See also T396824.