Update deprecation rules

[rhattersley]

Since the switch to semver the old two-release-or-six-months rule doesn't really make sense. This change simplifies it to just a one-release minimum.

(When someone chooses to install a major release is an independent question.)

[pp-mo]

I'm 👍 for this.
Any dissenters ??

[ajdawson]

👍 let's do it

[pp-mo]

Ooops, just spotted, the six-month/two-revisions part is mentioned again lower down ~Line 75
( can't link to a .rst ?)

[pp-mo]

I think we should be mentioning SemVer explicity.
That can be handled as a separate issue though.

[rhattersley]

Ooops, just spotted, the six-month/two-revisions part is mentioned again lower down ~Line 75
( can't link to a .rst ?)

Well spotted 👀 - thank you. I've updated that now.