Update to Ajv v7

[sdirix]

Update JSON Forms' Ajv dependency to v7.

ToDo

• Update to latest Ajv. See also Ajv as peer dependency #1557 and Ajv deprecated "errorDataPath" option. #1565.
• Check backwards compatibility (see also ajv-errors).
• Potentially add guide to migration manual.

[sdirix]

Also potentially remove time validation, see #1689