Fix TLS errors when using acme/autocert for local connections

[joohoi]

The upstream library golang.org/x/crypto/acme/autocert, a package that handles the Let's Encrypt certificate automation terminates incoming HTTP requests if the SNI (ServerName) field isn't in it's whitelist. For internal connection this domain is localhost which cannot be whitelisted for autocert.Manager to automatically try to handle certificates for.

This PR simply adds a ServerName setting to TlsClientConfig for connections made towards localhost, thus mitigating this issue, as the certificate requested from autocert.Manager indeed matches the domain in its whitelist.

This isn't an issue for non - Let's Encrypt setups either, and quite the opposite - in many cases should also be able to eliminate the need for InsecureSkipVerify.

Fixes: #5800

[codecov-io]

Codecov Report

Merging #5820 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5820      +/-   ##
==========================================
- Coverage   37.88%   37.87%   -0.01%     
==========================================
  Files         328      328              
  Lines       48255    48255              
==========================================
- Hits        18280    18278       -2     
- Misses      27345    27348       +3     
+ Partials     2630     2629       -1

Impacted Files	Coverage Δ
models/unit.go	0% <0%> (-14.29%)	⬇️

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[lafriks]

@joohoi please send backport to release/v1.7 branch