bump minSdkVersion to 26 (Android 8.0)

[mahibi]

came up because of
#4341 (comment)
#4338

I just calculated the percentage usage of android 7.0 and 7.1 for our user base.

For 7.0 it is 0,3%
For 7.1 it is 0,19%

We have the rule to ditch android version lower than 0,5%.

ToDo:

• remove checks for android versions older than api level 26
• bump minSdkVersion to 26
• update documentation at
  • https://docs.nextcloud.com/server/latest/admin_manual/installation/system_requirements.html#mobile-apps
  • https://nextcloud-talk.readthedocs.io/en/latest/user-requirements/#mobile-apps