fix: pages router metadata bugs with React 19 (#81733)
When we updated `experimental.strictNextHead` to be true in #65418, we
did not update all spots that would default to true in the case where
the value was omitted entirely.
This led to the default value not being correctly applied in pages
router, which resulted in duplicate metadata w/ React 19.
Since we made the flag the default, we can also probably clean up this
flag all together, but that can be done separately.
Fixes #81655
Fixes #81689
