all: end support for Windows 7, Windows Server 2008 in Go 1.21

[heschi]

Per discussion in #52188, Microsoft, Chrome, Git and others will stop supporting Windows 7 at the beginning of next year. We should follow suit. Windows Server 2008R2, which I understand we use mostly as a proxy for 7, will fall out of support at the same time.

On behalf of @golang/release, I propose we announce the end of support in the 1.20 release notes, and disable the builder for 1.21.

[bcmills]

(CC @golang/windows)

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

Windows 7 is clearly no longer supported by Microsoft or many others, so this seems fine.

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group