When the `ci_sources_pipelines` table was first introduced in GitLab 9.3
EE, the foreign key names generate for the table appeared to have been
calculated via a hash using the table name
`ci_pipeline_source_pipelines`. This led to a merge conflict and
confusion during a CE to EE merge in GitLab 10.0, which regenerated the
schema with the correct foreign key names.

Hence anyone who installed GitLab prior to 10.0 may have been seeded the
database with stale, incorrect foreign key names.

During the Great BigInt Conversion of 2021, the migrations assumed that:

1. The foreign key `fk_be5624bf37` existed for `ci_sources_pipelines`.
2. The index `index_ci_sources_pipelines_on_source_job_id` existed.

However, older installations may have had the foreign key under the
original `fk_3f0c88d7dc` and
`index_ci_pipeline_source_pipelines_on_source_job_id`, respectively.

To eliminate future confusion and migration failures, we now rename the
foreign key constraints and index to what they should be today.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/339844

Changelog: fixed