Stop building packages for openSUSE 15.1

openSUSE 15.1 EOL is on February 2021, which means 13.6 will be the last GitLab version with packages built for it. We should add a deprecation notice for it and announce in blog post.

Pages that will need update:

• https://docs.gitlab.com/omnibus/package-information/deprecated_os.html
• https://docs.gitlab.com/ee/install/requirements.html#supported-linux-distributions
• https://about.gitlab.com/install/