[TASK] Use modern HTML syntax for boolean attributes

[billdagou]

Fluid used the XHTML syntax to represent boolean values. This change adjusts
the TagBuilder to use the modern syntax. This change is considered non-breaking
because it results in the same interpretation of the browser. However, it will
not be backported to Fluid v4 to be safe.

Before:

<my:tagBased required="{true}" />
Result: <input required="required" />

After:

<my:tagBased required="{true}" />
Result: <input required />

[s2b]

What was the reason for the additional requireAttributeValues configuration? From my point of view, this isn't necessary, we can just switch to the alternative syntax.

[billdagou]

To switch between <input required /> and <input required="required" />, like for config.doctype = xhtml_* or config.doctype = html5 in TYPO3.