TYPO3 v13 compatibility

[skranzusch]

This merge allows v13 Instances of TYPO3 to use headless_powermail (PR #37 is included with this)

[tmotyl]

@skranzusch which powermail version are you testing this with?
Are these changes only for powermail v13 and TYPO3 v13 or should they be applied to other TYPO3/powermail versions combinations?
What is the reason for having different requirmeents in composer.json and ext_emconf?

[skranzusch]

@tmotyl I have tested with Powermail 13.0.3 and the changes are for TYPO3 v14 and Powermail v13. It reflects changes that came with Powermail 13 and would not be needed in prior versions. Of course I cannot predict future versions. But it has no backward compatibility. The important adaptions lay in a changed HMAC generation from TYPO3 13 core and a changed method signature in an extended class from Powermail.

The reason for differences in requirements between composer.json and ext_emconf where a mistake on my part. Minimal requirement should be v13 in both TYPO3 and Powermail. I have corrected that. Thanks for noticing that.

[tmotyl]

thanks! ok, so this change should go to a major release.
Lets first release v3.0.0 (as current ext_emconf has already v3) with current master v4.0.0 with this change.
what do you think @kitzberger ?