Add syntax validation for build with 50 tags to prevent pipeline creation

Release notes

GitLab values efficiency and is prioritizing reliability for GitLab.com in FY22. In 14.3 pipelines with more than 50 tags will not be created. You will receive an error for jobs that exceed 50 tags. For more information see our documentation.

Problem

As brought up by @ayufan it will be almost impossible to find why a build is not picked up by our new queuing system in case it has more than a 50 tags.

50 tags seems to be a reasonable limit as we double checked on production data.

Solution

We should add syntax validation mechanism that will prevent pipeline creation if someone is using a build with more than 50 tags.

1. Add validation to Gitlab::Ci::Config::Entry::Job
2. Throw an error if limit is reached
3. Add specs for it