Gitpod Image PHP 8.0 regressed to 7.4

[apolopena]

Describe the bug

The underlying Gitpod image regressed it's version of PHP from 8.0 back to 7.4
This breaks the new functionality added in the v1.5.0 release

Screenshots

Steps to reproduce

1. ensure the docker image cache is busted
2. set php version to 'gitpodlatest'

• You will see errors in the image build log about purging a version of PHP that doesnt exist
• Once the workspace has loaded you will see that: php -v outputs PHP 7.4.3

Expected behavior

gitpodlatest should always be PHP >= 8.0 once the workspace is running.

Additional information

This issue is essentially a patch for gitpod-io/workspace-images#714

Probably the best way to handle this for now is to add an option for installing php8.0 and a pre install check that will skip the PHP install process if the current version (in the Gitpod image) is the same as the target version to install.