[SPARK-31989][SQL] Generate JSON rebasing files w/ 30 minutes step

[MaxGekk]

What changes were proposed in this pull request?

1. Change the max step from 1 week to 30 minutes in the tests RebaseDateTimeSuite.generate 'gregorian-julian-rebase-micros.json' and generate 'julian-gregorian-rebase-micros.json'.
2. Parallelise JSON files generation in the function generateRebaseJson by using ThreadUtils.parmap.

Why are the changes needed?

1. To prevent the bugs that are fixed by [SPARK-31959][SQL][test-java11] Fix Gregorian-Julian micros rebasing while switching standard time zone offset #28787 and [SPARK-31986][SQL] Fix Julian-Gregorian micros rebasing of overlapping local timestamps #28816.
2. The parallelisation speeds up JSON file generation.

Does this PR introduce any user-facing change?

Yes

How was this patch tested?

By generating the JSON file julian-gregorian-rebase-micros.json.

[MaxGekk]

@cloud-fan @HyukjinKwon After merging this bug fix #28816 and make the rebase functions pure by #28824, the JSON file will have no diffs.

[SparkQA]

Test build #124010 has finished for PR 28827 at commit 900fe2c.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[MaxGekk]

@cloud-fan @HyukjinKwon I re-generated JSON files on my local laptop. The JSON files are the same as the committed files to master currently.

[SparkQA]

Test build #124133 has finished for PR 28827 at commit 69fd414.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

Merged to master.