Make DataPayload::new_owned() no longer const (#4456)

Technically breaking, but I think this constness was a mistake and we
don't use it internally (and there's very little reason for someone to
use it externally, the static stuff exists).

This is useful for #4449 ; it
allows us to reference statics in the constructor of `DataPayload()`
which lets us reference a *proper* sentinel rather than a made-up value
that will not work in WASM.

CHANGELOG.md

@@ -5,6 +5,9 @@
 - Components
   - `icu_properties`
     - Add `Aran` script code (https://github.com/unicode-org/icu4x/pull/4426)
+- Data model and providers
+    - `icu_provider`
+        - (Small breakage) `DataPayload::new_owned()` is no longer `const`, this was a mistake (https://github.com/unicode-org/icu4x/pull/4456) 
 - FFI:
     - All languages
       - Correctly handle invalid UTF8 (https://github.com/unicode-org/icu4x/pull/4353)

provider/core/src/response.rs

@@ -194,7 +194,7 @@ where
     /// assert_eq!(payload.get(), &local_struct);
     /// ```
     #[inline]
-    pub const fn from_owned(data: M::Yokeable) -> Self {
+    pub fn from_owned(data: M::Yokeable) -> Self {
         Self(DataPayloadInner::Yoke(Yoke::new_owned(data)))
     }