Remove doc scoped labels from templates

What does this MR do and why?

Updates the docs issue and MR templates to remove the docs:: scoped labels, per:

• technical-writing#1143 (closed)
• gitlab-com/content-sites/handbook!9054 (merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A