Create php-composer branch to test Composer v2 lockfiles

Problem to solve

The purpose of this issue is to create a branch in the php-composer test project to test a Composer v2 lockfile.

See this discussion for background.

Proposal

1. Create a branch in the php-composer project
2. Use a >= 2.0.0 release of Composer to generate a new composer.lock file
3. Add the new php-composer branch as a downstream QA test in gemnasium
4. Update the Tested Versions entry for Composer in the Obtaining dependendency information by parsing lockfiles table in the Dependency Scanning documentation to show that 2.x of Composer is also supported

User experience goal

Testing is improved for gemnasium

Documentation

Dependency Scanning documentation will be updated as part of this issue.

Availability & Testing

New tests will be added to the php-composer test project

What does success look like, and how can we measure that?

Tests are added to the php-composer project to confirm that Composer >= 2.0.0 lockfiles are supported

What is the type of buyer?

Enterprise Edition GitLab Ultimate

Is this a cross-stage feature?

No, this only affects ~"Category:Dependency Scanning"

/cc @NicoleSchwartz @gonzoyumo @fcatteau

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