Remove Spark 3 specific wordings in docs

[manuzhang]

Feature Request / Improvement

Some Spark documentations are specific to Spark 3, (e.g. Using Iceberg in Spark 3). I believe that's to distinguish Spark 2 and Spark 3 supports long time ago. Now we've supported Spark 4 and there should be no differences from Spark 3. It's time to remove those wordings.

Query engine

None

Willingness to contribute

• I can contribute this improvement/feature independently
• I would be willing to contribute this improvement/feature with guidance from the Iceberg community
• I cannot contribute this improvement/feature at this time

[jackylee-ch]

I’d like to work on this issue.