Fix GitLab-CI configuration on both 1.0.x and 2.0.x

Problem/Motivation

I'm trying to merge D10 support into the 1.0.x branch. However, the GitLab CI pipelines are now failing over there:

https://git.drupalcode.org/project/entitygroupfield/-/pipelines/50342

Unable to create pipeline
'phpstan' job needs 'composer' job, but 'composer' is not in any previous stage
'stylelint' job needs 'composer' job, but 'composer' is not in any previous stage
'eslint' job needs 'composer' job, but 'composer' is not in any previous stage

Steps to reproduce

Proposed resolution

Fix .gitlab-ci.yml on both branches to make it easy to test across multiple core versions, confirm the matrix testing in 2.0.x is still working how we want, and ensure tests are passing on both branches.

Remaining tasks

1. Revisit how GitLab CI is configured on all branches.
2. See if there's now a cleaner way to handle the matrix testing we want/need (in 2.0.x).
3. See if we can go back to relying on the upstream defaults (in 1.0.x) to better handle what core versions we're testing with.
4. Confirm all pipeline steps and tests are passing everywhere.

User interface changes

API changes

Data model changes