[ML] Adjust structure finder for Joda to Java time migration

[droberts195]

The ML file structure finder has always reported both Joda
and Java time format strings. This change makes the Java time
format strings the ones that are incorporated into mappings
and ingest pipeline definitions.

The BWC syntax of prepending "8" to these formats is used.
This will need to be removed once Java time format strings
become the default in Elasticsearch.

This commit also removes direct imports of Joda classes in the
structure finder unit tests. Instead the core Joda BWC class
is used.

[elasticmachine]

Pinging @elastic/ml-core

[droberts195]

This PR should not be merged until Ingest pipeline has been changed to accept Java time format strings. The necessary changes are being made in #37407. I will double check the changes in this PR once #37407 is merged.

[droberts195]

#37407 is now merged and I have updated this PR to account for it.

[benwtrent]

For my edification: why are some formats prefixed with a 8?

[droberts195]

For my edification: why are some formats prefixed with a 8?

The 8 prefix is a way of telling the ES mappings and ingest code that the format is a Java time format rather than a Joda time format during the transition period.

In ES version 6.7 Joda time formats are the default and Java time formats must be prefixed with 8.

In ES version 7.x Joda time formats will no longer be supported in ES but the 8 prefix will be optional for Java time formats for BWC. Closer to the 7.0 beta1 feature freeze there will be more changes to the ES time code to implement this, and I will have to raise another PR to modify the file structure finder again. (That future PR will be for 7.0 only.)