Use dns-lexicon's Client instead of deprecated providers

[fupduck]

lexicon.providers.* is deprecated and it is suggested to use lexicon.client.Client instead.
Therefore I tried to implement the new approach.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.36%. Comparing base (8dc9d21) to head (0cef292).

Files	Patch %	Lines
certbot_dns_hetzner/dns_hetzner.py	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
- Coverage   89.58%   89.36%   -0.23%     
==========================================
  Files           2        2              
  Lines          48       47       -1     
==========================================
- Hits           43       42       -1     
  Misses          5        5              

Flag	Coverage Δ
unittests	89.36% <80.00%> (-0.23%)	⬇️

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