add allow_html5 option to date and time FormType to disable HTML5 input type

[Form] added allow_html5 option to date and time FormType to disable HTML5 input type when widget is set to single_text

| Q                    | A
| --------------------- | ---
| Bug fix?          | no
| New feature?   | yes
| BC breaks?     | no
| Deprecations? | no
| Tests pass?    | yes
| Fixed tickets   | #6927
| License           | MIT
| Doc PR           | 

With this little patch we can have a single text widget without HTML5 date input type which is required when using some javascript date or time picker .