feat(datacenter, server-type): move available and recommended to server_type

[phm07]

See:

• https://docs.hetzner.cloud/changelog#2026-04-01-datacenter-deprecations
• feat(datacenter, server_type): move available and recommended to server_type hcloud-go#835

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.91%. Comparing base (58e80f9) to head (f5279f4).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/cmd/datacenter/describe.go	75.00%	1 Missing ⚠️
internal/cmd/servertype/list.go	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1384      +/-   ##
==========================================
+ Coverage   66.89%   66.91%   +0.02%     
==========================================
  Files         300      300              
  Lines       13487    13506      +19     
==========================================
+ Hits         9022     9038      +16     
- Misses       3567     3570       +3     
  Partials      898      898              

Flag	Coverage Δ
e2e	49.35% <3.84%> (-0.05%)	⬇️
unit	67.59% <100.00%> (+0.02%)	⬆️

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[lukasmetzner]

I have some suggestions on the coding style. I found the code to be hard to read, otherwise.

Other than that, this looks good once we bump hcloud-go and rebase this.