Change all instances of btn-small to btn-sm to keep things consistent

What does this MR do?

Addresses the issue added at https://gitlab.com/gitlab-org/gitlab-ce/issues/37460 Renames all instances of btn-small to btn-sm

Are there points in the code the reviewer needs to double check?

If the classes where completely duplicated before it should be ok, but if one of the classes had styles not present in the other there might be some changes. Is there an easy way to test visual changes across the whole app without trying to see all of the button instances manually?

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/37460