Update release schedule (after EOL of 1.10)

From <oak-dev@jackrabbit.apache.org>:

<https://jackrabbit.apache.org/oak/docs/release-schedule.html> currently says:

> trunk: every 8 weeks (~6 / year)
> 1.22: every 4 weeks (~12 / year)
> 1.8: every 8 weeks (~6 / year)
> 1.6: every 13 weeks (~4 / year)
> 1.4: every 26 weeks (~2 / year)
> 1.2: every 52 weeks (1 / year)

We currently release 1.22 and 1.8 way more frequently than needed. I'd like to change both to 4 per year.

(this doesn't mean that we can't more frequent releases, for instance in case of a major bug or a security issue)

So the new schedule would say:

> trunk: every 8 weeks (~6 / year)
> 1.22: every 13 weeks (~4 / year)
> 1.8: every 13 weeks (~4 / year)
> 1.6: every 26 weeks (~2 / year)
> 1.4: every 26 weeks (~2 / year)
> 1.2: every 52 weeks (1 / year)

(also note that 1.2 is planned to be retired this May).