Update gitlab-runner links to main in docs

In https://gitlab.com/gitlab-org/gitlab-runner/-/issues/27239 the
default branch of GitLab Runner is going to change to `main` which
already exists and being kept in sync with `master` until the full
migration is done.

In some cases I've added `/-/` path seperator to use the new URL
structure in GitLab.

reference https://gitlab.com/gitlab-org/gitlab-runner/-/issues/27239