W3C Errors because of missing type attribute

[Georg Ringer]

Even though w3c validation is not that important anymore there was a client request to fix errors

Error: A source element that has a following sibling source element or img element with a srcset attribute must have a media attribute and/or type attribute.

[From line 813, column 34; to line 813, column 211](https://validator.w3.org/nu/#l813c211)

 <picture><source srcset="/fileadmin/_processed_/3/3/csm_Screen_MB_PHONE_skew_bewerbung_71fd9e0992.png 1x, /fileadmin/_processed_/3/3/csm_Screen_MB_PHONE_skew_bewerbung_2ec251f74c.png 2x"><sourc

don't ask me about details of the validation - just wanna fix it - PR incoming