[10xx] SDET backports

[theopnv]

Purpose of this PR

• Backported Clean old files #5072
• Backported Fix draft reminder #5653
• Backported Update the templates pipelines #5627
• Updated readme to have the same as on master.
• Moved _tmp_postprocessing.yml to postprocessing_publish.yml to be in sync with master. Added 2021.2 to the list of supported unity releases, removed 2021.1 as it will soon go out of support.

---

Testing status

• 🟢 Opening this PR as non-draft to check the reminder works
• 🟢 Checking the templates pipeline is now part of the nightly (canceling the pipeline).
• 🟡 Checking the templates pipeline is green.
  --> Pipeline is failing because the 10.8.0 dependency packages were not yet released. It is unrelated to this PR.

[github-actions]

It appears that you made a non-draft PR!
Please convert your PR to draft (button on the right side of the page)
and cancel any jobs that started on Yamato.
See the PR template for more information.
Thank you!