refactor(primary-ip): make set-rdns consistent with other commands

[phm07]

Since hetznercloud/hcloud-go@41a4c5a Primary IPs support the RDNSSupporter interface, so the base.SetRdnsCmd can be used for Primary IPs to make them consistent with other subcommands.

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (f31bb6e) 45.46% compared to head (6c66854) 45.32%.

Files	Patch %	Lines
internal/cmd/primaryip/set_rdns.go	50.00%	4 Missing ⚠️
internal/cmd/primaryip/primaryip.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #686      +/-   ##
==========================================
- Coverage   45.46%   45.32%   -0.15%     
==========================================
  Files         179      179              
  Lines        7890     7855      -35     
==========================================
- Hits         3587     3560      -27     
+ Misses       3846     3842       -4     
+ Partials      457      453       -4     

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