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MINOR: [Go][FlightSQL] reference the ADBC drivers in README (#34418)



### Rationale for this change

It is not easy to find the Golang database/sql FlightSQL driver. This change anticipates that people will look for it here first.

### What changes are included in this PR?

Only README content.

### Are these changes tested?

No.

### Are there any user-facing changes?

No.

Authored-by: Jacob Marble <jacobmarble@gmail.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>