Add calendar and numberingSystem options to date/time

[macchiati]

This is as per the discussion in the WG, separating them out from #911

Two changes from that:

1. Used RECOMMENDED instead of Optional, as per Change standard/optional to required/recommended #946
2. I noticed also there that RECOMMENDED needs a maturity status, so added a placeholder for that, with Proposed. Since that is only in exploration/maintaining-registry.md, I presume it would be removed for release.

[aphillips]

I resolved the conflict. In doing so, I reordered the proposed options below the REQUIRED one and rephrased the text slightly to take advantage of the fact that REQUIRED and RECOMMENDED are 2119 keywords. Please have a look.

[macchiati]

LGTM. Shall I go ahead and merge?

[eemeli]

Looks good, pending the inline suggestion to drop the maturity level mention.

[eemeli]

At least the calendar and numberingSystem mentions should also be removed from this note:

message-format-wg/spec/registry.md

Lines 952 to 964 in 960f800

	> [!NOTE]
	> The following options do not have default values because they are only to be used
	> as overrides for locale-and-value dependent implementation-defined defaults.
	The following date/time options are **not** part of the default registry.
	Implementations SHOULD avoid creating options that conflict with these, but
	are encouraged to track development of these options during Tech Preview:
	- `calendar` (default is locale-specific)
	- valid [Unicode Calendar Identifier](https://cldr-smoke.unicode.org/spec/main/ldml/tr35.html#UnicodeCalendarIdentifier)
	- `numberingSystem` (default is locale-specific)
	- valid [Unicode Number System Identifier](https://cldr-smoke.unicode.org/spec/main/ldml/tr35.html#UnicodeNumberSystemIdentifier)
	- `timeZone` (default is system default time zone or UTC)
	- valid identifier per [BCP175](https://www.rfc-editor.org/rfc/rfc6557)

[aphillips]

@eemeli I will make a separate PR to remove the note.

[eemeli]

I'm fine with removing the note separately.

[mihnita]

For some reason I can review, but can't approve.
My only button is "Submit review"

So: approved