Allow updating single_cluster_routing in google_bigtable_app_profile

[modular-magician]

Fixes b/168053560 (updating certain bt app profile fields failed)

Terraform believed the subfields of single_cluster_routing were updatable and they should have been, but in practice Magic Modules didn't generate the write update handlers.

I don't believe updating to/from single_cluster_routing and multi_cluster_routing is possible, so this schema is a little misleading. However I believe it's an acceptable tradeoff:

• Since this and mcr are ExactlyOneOf, mcr's ForceNew will still trigger if the user changes which field is set
• MM doesn't support updating fields that are updatable while their parent field isn't
• MM doesn't support update masks for non-top-level fields (but specifying the top level field will update all of it's children)

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

bigtable: fixed the update behaviour of the `single_cluster_routing` sub-fields in `google_bigtable_app_profile`

Derived from GoogleCloudPlatform/magic-modules#3967

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