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

[phm07]

See https://docs.hetzner.cloud/changelog#2026-04-01-datacenter-deprecations

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.76%. Comparing base (8e36441) to head (b2b4029).

Files with missing lines	Patch %	Lines
hcloud/datacenters/domain.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #645      +/-   ##
==========================================
- Coverage   96.77%   96.76%   -0.02%     
==========================================
  Files          76       76              
  Lines        4035     4046      +11     
==========================================
+ Hits         3905     3915      +10     
- Misses        130      131       +1     

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