Update the export version to v0.6

[sphuber]

This should take care of a few things:

• Simplify the schema due to the migration of Django to JSONB for the attributes and extras by removing the data serialization dictionaries. This was only used for datetime objects and since they are no longer supported, the whole concept can and should be removed.
• Convert datetime objects from attributes and extras to isoformat strings, they are no longer supported
• Apply the data migration for old JobCalculation nodes that will be handled in Add data migration for old calculation jobs #2854

[CasperWA]

This is essentially an "umbrella" issue for:

• Issue Simplify export file format after JSONB migration is performed #2815: Simplify export file format
• Issue Add data migration for old calculation jobs #2854: Data migration for old JobCalculation nodes
• Project 11: Django JSONB

Furthermore, it should be noted that whenever the export version is upped, migration functions must be made (even if it is to just up the version number in the migrated archive) and tests for it, possibly including a new archive for aiida-export-migration-tests.

Finally, it should be noted that this may even be affected further by issue #1762.
However, this issue could also be resolved later by upping the export version again (to v0.7)-

[CasperWA]

I believe this issue can be closed now, correct @sphuber?

[sphuber]

Correct, this was fixed in PR #3090