chore: bump Spark 3.5 to 3.5.9

[andygrove]

Which issue does this PR close?

N/A — routine patch-version bump within the already-supported Spark 3.5 line.

Rationale for this change

Spark 3.5 support should track the latest Spark 3.5 patch release. Apache Spark
3.5.9 has been released (tag v3.5.9), so this moves Comet's Spark 3.5 build,
CI, and documentation from 3.5.8 to 3.5.9.

What changes are included in this PR?

• Bumps <spark.version> in the spark-3.5 Maven profile to 3.5.9. Spark 3.5.9
  makes no transitive dependency changes relative to 3.5.8 (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/3.5.8.diff to dev/diffs/3.5.9.diff and regenerates it
  against the v3.5.9 tag. Three hunks needed manual reapplication:
  • pom.xml — context drift only (ivy.version bumped upstream).
  • WriteDistributionAndOrderingSuite.scala — context drift only (upstream
    added TransformExpression to a neighbouring import).
  • KeyGroupedPartitioningSuite.scala — context drift plus one real change:
    3.5.9 adds a new collectAllShuffles helper matching on
    ShuffleExchangeExec. Under Comet the shuffle is a
    CometShuffleExchangeExec, so the new helper's assert(allShuffles.nonEmpty)
    call sites would fail. It now matches ShuffleExchangeLike, consistent with
    how the existing collectShuffles helper is already patched.
• Points the Spark SQL, Spark SQL writer, and Iceberg 1.9/1.10 CI jobs at 3.5.9,
  and updates dev/ci/compute-changes.py's spark_3_5 path filter to the new
  diff filename.
• Updates the user guide's supported-version table, the Spark version
  compatibility page, the Iceberg contributor guide, the setup-spark-builder
  action / reusable-workflow input descriptions, and the pip install pyspark==
  examples in the PyArrow UDF test scripts.

How are these changes tested?

• git apply --check dev/diffs/3.5.9.diff against a pristine v3.5.9 checkout
  passes with no rejects or fuzz.
• Verified the regenerated diff touches exactly the same 73 files as the 3.5.8
  diff, and that its only content differences from 3.5.8 are the context lines
  above plus the collectAllShuffles change.
• npx prettier --check on the modified markdown.
• Full verification is the Spark SQL CI job on this PR, which clones v3.5.9,
  applies the new diff, and runs the SQL test suites.

[mbutrovich]

Approved pending CI, thanks @andygrove!