Rename FormField Value() to getFormattedValue()

[emteknetnz]

Split off from #11449 (comment)

FormField::getValue() was added in #11422 when implementing FieldValidation on FormFields

getValue() is confusing alongside Value() which should really be called getFormattedValue() as the method will often return a formatted version $this->value, which seems like it's intended for use within templates. It also looks like it's used a lot inside FormFields as $this->Value(), $this->getValue() should probably be used instead.

Acceptance criteria

• Rename FormField::Value() to FormField::getFormattedValue()
• Update FormFIeld code that used $this->Value() to use $this->getValue() instead where appropriate