IdP release schedule information

[Cantor, Scott]

Bringing this to general attention, as we finally get some ducks lined up:

https://shibboleth.atlassian.net/wiki/x/AwBqEAE 

We'll try and maintain that page going forward but the important information is at the top, you can read the rest if you care about the "why" parts of this.

Basically we are forced to adjust our release schedule and compress our timelines a bit based on Spring's shifting plans for support of their OSS edition so we have been discussing internally and doing some testing to firm up the plan.

We're releasing a patch this week (5.1.5) to get a new Spring version in place, and moving to what we hope will be a more regular release schedule for the IdP core in Q4-Q1 of each year (i.e. 5.2 should be out late this year or early next and we will try and stick to that cadence apart from exigent patches).

One of the common asks of us for a long time has been regular scheduling of releases, so hopefully this is helpful even if the reasons aren't.

The most notable point is that 5.1.x will be EOL next July out of necessity and so we're committed to getting 5.2 out with Spring 7 as soon as practical to extend the support life out into 2027.

-- Scott