Bugs and regression triage and prioritization process

This MR aims to simplify the triage process for both ~bug and regression. And a follow up to https://gitlab.com/gitlab-org/gitlab-ce/issues/48518

High-level changes

• Add a Defect section to capture both bugs and regressions.
• Added a Bugs section with steps on how to triage bugs
• Clarified and simplified the Regression section and also steps on how to triage regressions.
  • Spell out clearly what is the special regression case for Last recent monthly release and Current release candidates
  • Add examples for the above
• Introduce the use of regression:xx.x label which will be used by other mechanisms in the release process.
• Persist regression labels since this label is for categorization and not time bound.

@godfat @rymai @markglenfletcher @ddavison @smcgivern @DouweM @bikebilly lets start with you folks first.