Simplify the registry auto-detection

[rdimitrov]

The following PR aims to fix 2 issues:

• the API server was returned a cached version of the registry (changes via the API were made available after the server restarted)
• the auto-detection logic was wrongly relying on an /openapi spec being served, so this simplifies it

[codecov]

Codecov Report

❌ Patch coverage is 59.21053% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.12%. Comparing base (147d5f5) to head (95ff645).

Files with missing lines	Patch %	Lines
pkg/config/registry.go	67.85%	9 Missing and 9 partials ⚠️
pkg/api/v1/registry.go	55.55%	3 Missing and 1 partial ⚠️
pkg/registry/api/client.go	0.00%	4 Missing ⚠️
pkg/registry/provider_api.go	0.00%	3 Missing ⚠️
pkg/registry/provider_remote.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2976      +/-   ##
==========================================
+ Coverage   56.09%   56.12%   +0.03%     
==========================================
  Files         328      328              
  Lines       32376    32425      +49     
==========================================
+ Hits        18160    18200      +40     
+ Misses      12676    12671       -5     
- Partials     1540     1554      +14     

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