Change description for the ~meta label to be more generic

The current definition of the meta label is:

Issues relating to the GitLab Community Edition Project itself, for example issue triage or labeling policies.

Now that the label has been promoted at group level, it may create some confusion when used in other projects like gitlab-ee. We should reconsider the meta description.

We could change it in:

Issues relating to GitLab flows, for example issue triage or labeling policies.

There is also something that is still out of the original scope: using meta for general discussion about features (sort of epic replacement before we had epics. What about that? Do we want to keep this scope as well? In case, we should mention in the description.