`meta-theme-color`: reconcile BCD and caniuse

[ddbeck]

This feature's inconsistent between caniuse (mapped) and BCD. Since it's a SHOULD not MUST behavior, this ought to be reworked as a kind of "hint" feature.

See also:

• Safari 26 uses <meta name="theme-color"> only for web apps mdn/browser-compat-data#28503
• Safari 26 doesn't use theme-color anymore Fyrd/caniuse#7366