Add .gitlab-ci.yml

Tried to opt in to testing against 10.3.x with OPT_IN_TEST_NEXT_MINOR but it appears to still be using 10.2.4-dev

This is working with _TARGET_CORE, we'll just have to remove it once 10.3.x is the default.

Opened 📌 Add phpstan.neon and phpstan-baseline.neon Active to address phpstan issues.

Settled on _TARGET_CORE: "10.3.x-dev||$CORE_SUPPORTED" which will pick the higher of 10.3.x-dev or whatever is currently supported.

Updated proposed resolution and added related issues

Thanks, got confirmation that should be good to add these type of files to the 1.0.x splits. Was worried it would be considered deviating too much.

Automatically closed - issue fixed for 2 weeks with no activity.