chore: bump Spark 4.0 to 4.0.4

[andygrove]

Which issue does this PR close?

N/A — routine patch-version bump within the already-supported Spark 4.0 line.

Rationale for this change

Spark 4.0 support should track the latest Spark 4.0 patch release. Apache Spark
4.0.4 has been released (tag v4.0.4), so this moves Comet's Spark 4.0 build,
CI, and documentation from 4.0.2 to 4.0.4 (skipping 4.0.3).

What changes are included in this PR?

• Bumps <spark.version> in the spark-4.0 Maven profile to 4.0.4. Spark 4.0.4
  makes no transitive dependency changes relative to 4.0.2 (scala.version,
  parquet.version, slf4j.version, jetty.version, hadoop.version and
  arrow.version are all unchanged), so no other profile properties move.
• Renames dev/diffs/4.0.2.diff to dev/diffs/4.0.4.diff and regenerates it
  against the v4.0.4 tag. Three hunks needed manual reapplication, all pure
  import-line context drift with no behavioural change:
  • DataFrameWindowFunctionsSuite.scala — upstream added a
    catalyst.trees.UnaryLike import between the patched neighbours.
  • KeyGroupedPartitioningSuite.scala — upstream added RDDScanExec to a
    neighbouring import.
  • WriteDistributionAndOrderingSuite.scala — upstream added
    TransformExpression to a neighbouring import.
• Updates the two version references inside the diff's HiveUDFDynamicLoadSuite
  comment. hive-test-udfs.jar is still stripped from the 4.0.4 release source
  tag (only a README remains under sql/hive/src/test/noclasspath/), so the
  IgnoreCometSuite mixin is still required.
• Points the Spark SQL CI job and the PyArrow UDF job's pyspark pin at 4.0.4,
  and updates dev/ci/compute-changes.py's spark_4_0 path filter to the new
  diff filename.
• Updates the user guide's supported-version table and the Spark version
  compatibility page.

How are these changes tested?

• git apply --check dev/diffs/4.0.4.diff against a pristine v4.0.4 checkout
  passes with no rejects or fuzz.
• Verified the regenerated diff touches exactly the same 88 files as the 4.0.2
  diff, and that its only content differences from 4.0.2 are the context lines
  and comment version strings above.
• npx prettier --check on the modified markdown.
• Full verification is the Spark SQL CI job, which is label-gated for Spark 4.0
  (run-spark-4.0-tests) — that label should be applied before merge.

[mbutrovich]

Approved pending CI, thanks @andygrove!