Make pipeline subscriptions downstream limit configurable

Release notes

TBD

Problem to solve

The pipeline subscriptions feature currently has a limit of 2 for both upstream and downstream projects which is too restrictive and not configurable when there is a need to increase this limit for greater usability.

Proposal

Make the pipeline subscriptions feature's downstream limit a configurable setting at the project level.

Further details

Expand for original details when this issue was initially opened.

Currently, pipeline subscriptions have the following limit:

The maximum number of upstream pipeline subscriptions is 2, for both the upstream and downstream projects.

I would like to ask that downstream limit is relaxed. I want to use pipeline subscriptions to rebuild Docker images when their parent image (repository) gets rebuild (gitlab-foss#31930 (closed)), but with limit of 2, this is not possible because only two images can be configured like that against a common base Docker image. I understand the upstream limit, but is not that limit enough to limit things here? I mean, one can already configure unlimited triggers manually, so this downstream limit does not really prevent anything, it just make the feature hard to use, or even useless.