dnsapi/ionos: Update API to use POST instead of PATCH for adding TXT records

[lbrocke]

The IONOS DNS API now supports a POST route for adding records. Therefore checking for already existing records and including them in a PATCH request is no longer necessary. This does not affect functionality but rather simplifies the dns_ionos_add() function.

Docker/MacOS and Windows DNS tests are passing, FreeBSD fails due to an package installation error unrelated to this code change:

[..]
/usr/bin/ssh -t freebsd
Pseudo-terminal will not be allocated because stdin is not a terminal.
Warning: no access to tty (Bad file descriptor).
Thus no job control in this shell.
Installing pkg-1.16.3...
Newer FreeBSD version for package pkg:
To ignore this error set IGNORE_OSVERSION=yes
- package: ***2000
- running kernel: ***1000
Ignore the mismatch and continue? [y/N]: 
Failed to install the following 1 package(s): /tmp//pkg.txz.6vPgxn
Bootstrapping pkg from pkg+http://pkg.FreeBSD.org/FreeBSD:12:amd64/quarterly, please wait...
Verifying signature with trusted certificate pkg.freebsd.org.2013102301... done
Error: The process '/usr/bin/ssh' failed with exit code 1

[Neilpang]

rebase your code to latest dev branch first. the freebsd is fixed.

[lbrocke]

Done, tests are passing now.