CLDR-16933 Allow names for noon, midnight to match adjacent non-fixed dayPeriods; add tests & spec note

[pedberg-icu]

CLDR-16933

• This PR completes the ticket.

• Test adjustments needed to resolve an error in Ukrainian and perhaps also needed for other locales, per discussion in ticket:
  • Let midnight match a non-fixed period that starts at, ends at, or contains midnight (both 00:00 and 24:00)
  • Let noon match a non-fixed period that starts at, ends at, or contains noon (or just before noon);
• Add unit tests for this and for some other dayPeriodRuleSet name allowances we are already making.
• Add a spec note about formatting noon and midnight; these may not need an explicit numeric time, unless the same name is used for other dayPeriods.

ALLOW_MANY_COMMITS=true