Update all references for PHP7 to 8

[oceanicpanda]

semver:minor
@TisVictress I tried to run the Ruby tests according to https://github.com/paketo-buildpacks/php-dist/blob/main/dependency/actions/compile/binary-builder/README.md however the instructions seem a little broken and I've never used Ruby and copying the commands I could see into the terminal just gave me exceptions I couldn't figure out. Can you please shed some light here?

Summary

Since PHP7 no longer seems to be supported tests should be updated with minimum support versions and all others references should be removed/updated.

Checklist

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• [ n/a] I have added an integration test, if necessary.
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[sophiewigmore]

Hey! I'd recommend using the instructions here to compile the dependency: https://github.com/paketo-buildpacks/php-dist/blob/main/dependency/actions/compile/README.md
The code inside of that binary-builder directory is famously hard to run locally. @TisVictress is actually working on refactoring that code in #502 already!

[sophiewigmore]

@oceanicpanda can we close out this PR?

[oceanicpanda]

yep, sorry have been mega busy of late, will try and get back to it