Add experimental to datetime's bench features

[Nerixyz]

Both benchmark suites in the datetime component use the CompositeFieldSetSerde which is under the experimental feature. As a result, running cargo bench -F bench --manifest-path components/datetime/Cargo.toml will not run any benchmark. This currently causes CI to fail on the main branch (e.g. https://github.com/unicode-org/icu4x/actions/runs/12279258426/job/34262931118).

This PR adds the experimental feature to the bench feature. Alternatively, the workflow could specify additional features to pass to a component's benchmarks. However, when running the benchmarks locally, one would always need to specify the experimental feature.

[CLAassistant]

All committers have signed the CLA.

[sffc]

Thank you!