Changing date ingest processor to Java time also changes date index naming

[droberts195]

The date ingest processor was changed to allow Java time formats prefixed with 8 in #37407, but the default time format is still Joda after this change. To match what has been done for mappings in version 7 the default time format for ingest should be Java time, so that formats with and without an 8 prefix are treated identically.

I opened #37934 to make this change (as I want to test that the ML file structure finder works with it in v7). However, while making this change I found that changing the default for date processing in ingest also affects index naming with the date index name processor. After a brief chat with @spinscale I am raising this issue for further discussion of whether extra measures need to be taken regarding index naming by the date index name processor through the 6.x -> 7.x upgrade. I can imagine it might cause problems for Beats?

Relates #27330

[elasticmachine]

Pinging @elastic/es-core-infra

[droberts195]

/cc @elastic/beats

[jsoriano]

I have created an issue in Beats to track tasks around this elastic/beats#10433

[droberts195]

We discussed this in the ES core infra meeting and it seems that there is no particular problem with the date index name processor, so I will close this issue.