[andygrove]

Which issue does this PR close?

Part of #4329.

Rationale for this change

#7521 removes the spark-3.4 Maven profile and stops running Spark 3.4 in CI, but leaves the Spark 3.4 shim source sets behind. Nothing builds them now, so they are dead code that has to be kept in step with the other shims. This is the follow-up that deletes them.

What changes are included in this PR?

• Delete spark/src/main/spark-3.4/ and spark/src/test/spark-3.4/. Every file in them has a counterpart in spark-3.5/ or is a 3.5-only addition there, so the spark-3.5, spark-3.x and spark-4.x source sets are unchanged.
• Remove the spark-3.4 path entries from dev/ci/compute-changes.py. They only excluded edits to the deleted directory from triggering other jobs.
• Update comments that described 3.4 (ShimCometMapInBatch, GenerateDocs).
• Update the contributor and user docs that described Spark 3.4 behavior or listed spark-3.4 as a shim directory, and redraw shim_pattern.svg with the 3.5 and 4.x editions.

Deliberately not touched:

• The native get_json_object_spark34 function. The Spark 3.4 shim was its only caller, but it is a named behavior flag that Rename Cast.is_spark4_plus and get_json_object_spark34 to describe the behavior they select #6891 already tracks renaming, so it is better handled there.
• dev/diffs/3.4.3.diff and dev/diffs/iceberg/1.8.1.diff, as in chore: remove Spark 3.4 Maven profiles and stop running Spark 3.4 CI #7521.
• The "3.4.3" cases in the version-parsing unit tests and the bit_count_types.sql interpreted-mode workaround. They are harmless and can go in a later cleanup.
• The expression-audit pages and changelogs, which record what was true for each Spark version at the time.

How are these changes tested?

• python3 dev/ci/check-ci-config.py, check-versioned-source-parity.py and check-suites.py pass.
• prettier --check passes on the changed Markdown, and shim_pattern.svg parses as XML.
• Not run: a Maven build. No remaining profile compiles the deleted directories, and the only Scala changes are comments. CI on this PR exercises the remaining profiles.

[mbutrovich]

Thanks @andygrove!