Unable to validate iso19139 records which use regions keywords.

[ianwallen]

Describe the bug
Unable to properly validate records which use regions keywords.

To Reproduce
Steps to reproduce the behavior:

1. Create iso19139 record and assign a keyword from the regions
   
2. Validate the record'
3. See the following error with validation
   

Expected behavior
Expecting to be able to validate the record.

Desktop (please complete the following information):

• Browser chrome
• GeoNetwork Version 3.12.10/4.2.5.snapshot
• Server Application jetty

Additional context
Issue could have also been created from the following PR
#6972

It seems like the dates in the following file are incorrect.

core-geonetwork/web/src/main/webapp/WEB-INF/data/config/codelist/external/thesauri/place/regions.rdf

Lines 14 to 15 in 9b7bd67

	<dcterms:issued>2015-07-17 12:00:00</dcterms:issued>
	<dcterms:modified>2015-07-17 12:00:00</dcterms:modified>

So we could simply update the date on that thesaurus.
Or we could enhance that PR and add better date handling

Not sure what the recommended solution would be?

[fxprunayre]

Updated the date in the default thesaurus.
#7208 for the coming release.

Not sure what the recommended solution would be?

As we now set the dates in ISO records, maybe we should only accept valid ISO date format from thesaurus ?