Remove Gmail reference in regard to OAuth2 (and fix typos) in README.md

[gerryfrancis]

Referenced to issue deltachat/deltachat-android#3233 .

[iequidoo]

Warning: Permanently added 'mailadm.testrun.org' (ED25519) to the list of known hosts.
***@mailadm.testrun.org: Permission denied (publickey).
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: unexplained error (code 255) at io.c(231) [sender=3.2.7]

mailadm.testrun.org doesn't trust the build machine's public key apparently.

[missytake]

Warning: Permanently added 'mailadm.testrun.org' (ED25519) to the list of known hosts.
***@mailadm.testrun.org: Permission denied (publickey).
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: unexplained error (code 255) at io.c(231) [sender=3.2.7]

mailadm.testrun.org doesn't trust the build machine's public key apparently.

no, mailadm.testrun.org was taken down, and we forgot to migrate this machine's preview setup. So atm it's basically expected to fail, need to fix it at some point.