Clean up all mention of tech preview etc.

[aphillips]

PR to follow. action per 2024-11-11 call

[aphillips]

Done. Nothing to see here.

[eemeli]

Not quite. We still have this (#603):

message-format-wg/spec/formatting.md

Lines 537 to 540 in 0e9dca8

	> In the Tech Preview, feedback from users and implementers is desired about
	> whether to relax the requirement that such a "fallback _variant_" appear in
	> every message, versus the potential for a _message_ to fail at runtime
	> because no matching _variant_ is available.

and this:

message-format-wg/spec/registry.md

Lines 1078 to 1082 in 0e9dca8

	> True time zone support in serializations is expected to coincide with the adoption
	> of Temporal in JavaScript.
	> The form of these serializations is known and is a de facto standard.
	> Support for these extensions is expected to be required in the post-tech preview.
	> See: https://datatracker.ietf.org/doc/draft-ietf-sedate-datetime-extended/