[Task]: Deprecate Python native SpannerIO

[Abacn]

What needs to happen?

Beam Python SDK has two SpannerIO, a native one in io/gcp/experimental and a xlang wraper in io/gcp. The former one was long marked "experimental" and less exercised by both test and adoptions. Relevant issues: #20529 #35850. As part of I propose deduplicate Python SpannerIO in upcoming Beam 3. Before that, mark Python native SpannerIO as deprecated.

Issue Priority

Priority: 2 (default / most normal work should be filed as P2)

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[ahmedabu98]

@Abacn should we mark this as fixed?

[liferoad]

@Abacn should we mark this as fixed?

TODO: remove this from the code with Beam 3.0

[Abacn]

we already marked it for deprecation (scoping done), so move to Milestone 3.