Removal of artifacts:report:cobertura keyword
### Problem to solve
In https://gitlab.com/groups/gitlab-org/-/epics/6900, we specify the removal of the `artifacts:report:cobertura` keyword.
### Proposal
Remove `artifacts:report:cobertura` keyword:
- `cobertura` keyword: https://gitlab.com/gitlab-org/gitlab/blob/18cbbc260912e5a50f507659c53a08df7ac711f1/lib/gitlab/ci/config/entry/reports.rb#L48-L48
- validation across `cobertura` and `coverage_report`: https://gitlab.com/gitlab-org/gitlab/blob/18cbbc260912e5a50f507659c53a08df7ac711f1/lib/gitlab/ci/config/entry/reports.rb#L55-L55.
### Workaround after removal
If you are coming here to see that this feature has been removed and are wondering what the replacement keyword is, here is a new example:
```yaml
artifacts:
reports:
coverage_report:
coverage_format: cobertura
path: coverage/cobertura-coverage.xml
```
issue