property naming issue: 'readOnly' or 'readonly'

[ilhan007]

Describe the documentation issue
Several components have the property "readonly", but it is written differently.
It i called "readonly" in ui5-input and ui5-datepicker.
It i called "readOnly" in ui5-checkbox and ui5-radiobutton.

Expectation
As there is no difference in the purpose of the property,
we should stick to single name, either "readonly", or "readOnly"

Side notes
There is native "readonly" property already (and aria-readonly).

Affected components
ui5-input, ui5-datepicker, ui5-checkbox, ui5-radiobutton