[SPARK-30423][SQL] Deprecate UserDefinedAggregateFunction

[erikerlandson]

What changes were proposed in this pull request?

• Annotate UserDefinedAggregateFunction as deprecated by SPARK-27296
• Update user doc examples to reflect new ability to register typed Aggregator[IN, BUF, OUT] as an untyped aggregating UDF

Why are the changes needed?

UserDefinedAggregateFunction is being deprecated

Does this PR introduce any user-facing change?

Changes are to user documentation, and deprecation annotations.

How was this patch tested?

Testing was via package build to verify doc generation, deprecation warnings, and successful example compilation.

[cloud-fan]

OK to test

[SparkQA]

Test build #116650 has finished for PR 27193 at commit ea3fcd9.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
• public static class Average implements Serializable
• public static class MyAverage extends Aggregator<Long, Average, Double>
• case class Average(var sum: Long, var count: Long)

[cloud-fan]

thanks, merging to master!