### What changes were proposed in this pull request?
This PR renames `SupportsStreamingUpdate` to `SupportsStreamingUpdateAsAppend` as the new interface name represents the actual behavior clearer. This PR also removes the `update()` method (so the interface is more likely a marker), as the implementations of `SupportsStreamingUpdateAsAppend` should support append mode by default, hence no need to trigger some flag on it.
### Why are the changes needed?
SupportsStreamingUpdate was intended to revive the functionality of Streaming update output mode for internal data sources, but despite the name, that interface isn't really used to do actual update on sink; all sinks are implementing this interface to do append, so strictly saying, it's just to support update as append. Renaming the interface would make it clear.
### Does this PR introduce _any_ user-facing change?
No, as the class is only for internal data sources.
### How was this patch tested?
Jenkins test will follow.
Closes#29693 from HeartSaVioR/SPARK-32831.
Authored-by: Jungtaek Lim (HeartSaVioR) <kabhwan.opensource@gmail.com>
Signed-off-by: Jungtaek Lim (HeartSaVioR) <kabhwan.opensource@gmail.com>
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