Decide when Drupal 7 (and Drupal 8 and later) should enter the LTS and security-support-only phase

Some unfinished business from #2135189: Proposal to manage the Drupal core release cycle is that it's not clear when Drupal 7 should enter the LTS (long term support) phase (only bugfixes and security fixes allowed), and when it should enter the final support phase in which only security fixes are allowed.

Based on the diagram in that issue:

• Drupal 7 would have already been in the LTS phase for a while (we missed the boat on that).
• Drupal 8 would enter LTS around the time Drupal 9 is opened for development. Sites using Drupal 8 don't get new features anymore until Drupal 9 is stable and they actually decide to upgrade to it.
• Drupal 7 would enter the security-support-only phase 3 months after Drupal 8 enters the LTS, and Drupal 8 would enter that phase 3 months after Drupal 9 enters the LTS.

But there wasn't a whole lot of discussion of the above - and again, for #1 we missed the boat (already missed it before that issue even existed).

Some competing goals in determining these dates include:

1. Letting people who only want to use LTS releases still get to use the new version of Drupal sooner rather than later.
2. Not having too long of a period where the current release of Drupal core has stopped getting new features, but the next release of Drupal core is nowhere near the point where it will be used on actual sites.

We have to figure this all out at some point, but not necessarily right away - hence priority = "major" for now.

Decisions to make:

1. When does D7 enter LTS?
   • Current policy says it already was, but this isn't actually true.
   • Proposal from Dries and D8 committers: once D7/8 Migration is fully supported.
2. When does D7 LTS end and security-only support begin?
   • Current policy: When D8 LTS is available.
3. When does D8 LTS begin?
   • Current policy: next scheduled minor after 9.0.x opens for development
   • Proposed modification: When 9.0.0 is released. (Illustrations in #19.)
4. When does D8 LTS end and security-only support begin?
   • Current policy: when D9 LTS is available.
5. When is D7 EOL? Disucssed in #2608496: [policy, no patch] Drupal 9 release timing and Drupal 7/8 EOL
6. When is D8 EOL? Disucssed in #2608496: [policy, no patch] Drupal 9 release timing and Drupal 7/8 EOL

Proposed resolution

D7 LTS should start once the D7/8 migration path is fully supported.
D7 LTS should end once D8 LTS begins, same as the current policy. This is essential for LTSes to meet the need they're intended for.
D8 LTS should be released at the same time as 9.0.0.
D8 LTS should end once D9 LTS begins.
D9 LTS should end once D10 LTS begins.