install(EXPORT): Drop REQUIRED from exported find_dependency calls

In commit c6e6861e63 (install(EXPORT): Export find_dependency() calls,
2023-11-07, v3.29.0-rc1~439^2~1) we made the calls `REQUIRED`.  However,
a dependency is only required if the dependent package is required.
`find_dependency` already forwards the `REQUIRED` mark, and also already
marks the dependent package as not found if the dependency is missing.

Fixes: #25756