build(java): bump Spark 4 from 4.1.1 to 4.1.2

[jackylee-ch]

Rationale for this change

Spark 4.1.2 is the latest maintenance release of the 4.1 line on Maven Central,
containing bug fixes over 4.1.1. Keeping the Spark 4 dependency current picks up those
fixes for the vortex-spark_2.13 builds. Companion to #8812, which bumps the Spark 3
line.

What changes are included in this PR?

A one-line version bump in java/gradle/libs.versions.toml: spark4 = "4.1.1" →
spark4 = "4.1.2". No code changes.

Verified locally against 4.1.2 with :vortex-spark_2.13:test: 120 tests ran, all
passed except VortexDataSourceS3MockTest, which failed only because the local Docker
daemon was not running (Testcontainers infrastructure requirement, unrelated to the
Spark version).

What APIs are changed? Are there any user-facing changes?

None. Spark 4.1.2 is a drop-in maintenance upgrade; no connector code or public API
is touched.

[codspeed-hq]

Merging this PR will not alter performance

✅ 1670 untouched benchmarks
⏩ 53 skipped benchmarks¹

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_{Comparing jackylee-ch:spark-4.1.2-upgrade (b1ab074) with develop (60c1227)²}

Footnotes

1. 53 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on develop (3513367) during the generation of this report, so 60c1227 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩