RFR: Cleanups and fixes

[Jesper Wilhelmsson]

Various cleanups and fixes:
* Removed the "ToDo" from the README - it's been there from the start and had some random things suggested for improvement at the time but it hasn't guided the work or what we have focused on in the guide. It also wasn't updated when changes were made so by now it was mainly outdated.
* Removed the mention of LTS releases since this is not an OpenJDK concept.
* Updated Sponsor links in the context of sponsoring a change, the links were referring to the description of sponsoring a Project.
* Added a note on using N-pool values for FixVersion.
* Cleaned up description of AffectsVersion.
* Removed "Things to keep in mind when requesting an improvement" which is already covered in [Contributing to an OpenJDK Project](https://openjdk.org/guide/#contributing-to-an-openjdk-project)
* Changed backout process to not use sub-tasks.
* Fixed minor typos etc.

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Commit messages:
 - Don't use sub-task for backout
 - Add comment when moving issue
 - Fix typo
 - Don't fix in released versions
 - Cleaning up Affects Version
 - Removed Todo
 - Cleaned up sponsor links

Changes: https://git.openjdk.org/guide/pull/159/files
  Webrev: https://webrevs.openjdk.org/?repo=guide&pr=159&range=00
  Stats: 60 lines in 8 files changed: 5 ins; 30 del; 25 mod
  Patch: https://git.openjdk.org/guide/pull/159.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/159/head:pull/159

PR: https://git.openjdk.org/guide/pull/159