fix(instance_server): detach pn on server deletion

[Codelax]

Delete private-nic before server when managed by instance_server resource.
Useful for the workflow:

• Delete Server
• Delete PN

It would fail because the PN was not empty yet

[codecov-commenter]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (d6150ef) 73.36% compared to head (a7a8b00) 73.32%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2179      +/-   ##
==========================================
- Coverage   73.36%   73.32%   -0.05%     
==========================================
  Files         212      212              
  Lines       33262    33292      +30     
==========================================
+ Hits        24403    24410       +7     
- Misses       6764     6788      +24     
+ Partials     2095     2094       -1     

Files	Coverage Δ
scaleway/resource_instance_server.go	73.86% <40.00%> (-0.27%)	⬇️

... and 4 files with indirect coverage changes

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