[roll] Roll fuchsia [build] Re-enable deprecation warnings as errors

https://fxrev.dev/987642 made `deprecated-declarations` warnings errors
except when using experimental C++ versions. The exception logic assumed
that GN arg `experimental_cxx_version` was only defined when overridden
and was not future proof against the default C++ version changing.

https://fxrev.dev/1093351 increased the default version to 20, and
because the default version is defined by the default value of
`experimental_cxx_version`, this activated the exception in default
build configurations.

To fix this and avoid problems when increasing the default version in
the future, define the default version independent of
`experimental_cxx_version`, which now defaults to `false`, and change
the exception logic to be applied whenever `experimental_cxx_version` is
overridden.

Note that setting `experimental_cxx_version` to the default version in
`fx args` will apply the exception even though it is unnecessary.
Avoiding this would require exposing `_default_cxx_version` or another
variable, which seems undesirable.

Original-Bug: 324656569
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1150672
Original-Revision: 8f42e43c1b1df050beeaff0c617c4dd6c83184f6
GitOrigin-RevId: 486b07db6ab982067c6738b848605c0d43222f22
Change-Id: I39dee4b957883cea19ab258df657efedb6cb542b