Support FLOAT32 type in Spanner

[arawind]

Adds support for the newly added FLOAT32 type in Google Cloud Spanner.

Since FLOAT was already mapped to FLOAT64, this change modifies the existing mapping to FLOAT -> FLOAT32 instead.

Addresses #30700.

Please ignore the commit bumping the BOM version to 26.36.0. This is already being done as part of #30868

---

[github-actions]

Checks are failing. Will not request review until checks are succeeding. If you'd like to override that behavior, comment assign set of reviewers

[arawind]

assign set of reviewers

(The single failure is due to a socket timeout, which seems flaky as it wasn't there in the previous run; I'll anyway rerun the tests by rebasing on HEAD)

[github-actions]

Assigning reviewers. If you would like to opt out of this review, comment assign to next reviewer:

R: @damondouglas for label java.
R: @damondouglas for label io.
R: @nielm for label spanner.

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

The PR bot will only process comments in the main thread (not review comments).

[nielm]

LGTM