[Bug]: Unpin spanner client version until the version from libraries-bom catches up

[shunping]

What happened?

context: #36995

Currently, the spanner client version is pinned at 6.104.0 for fixing an issue in the version from the current libraries-bom (#35868). Once we have a newer version of libraries-bom and it moves beyond this spanner client version, we will need to unpin this.

Issue Priority

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

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