[Investigation] Split out mergeable/mergeable_state into more cohesive methods

When trying to move some code from mergeable -> mergeable_state, we had an incident occur (gitlab-com/gl-infra/production#6788 (closed)). This incident was due to mergeable_state being used to check if we can run a merged result pipeline. gitlab-com/gl-infra/production#6788 (comment 905268171) explains further.

mergeable_state is ambiguous in naming, and the assumption here is that it is can be merged, and then we have mergeable which could imply the same checks as mergeable_state. mergeable calls mergeable_state so mergeable_state is a subset of checks of mergeable

Therefore, this issue is to investigate the different uses of mergeable_state and potentially split it out into different methods, for example, a new method that checks if it is mergeable on a git level.

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