Rename remote mirrors and mirrors to push and pull mirrors

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I've brought this up in https://gitlab.com/gitlab-org/gitlab-ee/issues/2954 before. In reading and reviewing code, I frequently get mixed up with our terminology of "remote mirrors" and "mirrors". Most recently we introduced a bug because of this mixup (https://gitlab.com/gitlab-org/gitlab-ce/issues/51725).

@DouweM argues this isn't the right term in https://gitlab.com/gitlab-org/gitlab-ee/issues/2954#note_35847814, but I've seen that the community generally understands "pull" and "push" mirror quite well (e.g. https://gitlab.com/gitlab-org/gitlab-ce/issues/18732#note_96811837).

/cc: @jramsay