fix(server): accept HTTP/1.1 for backward compatibility

[agaudreault]

Some client such as curl will use http/1.1 by default and only supporting the http2 protocol is a breaking change for these clients. Also, the http2 is not supported in Argo currently for HTTPS as far as I can see, only for grpc.

We must support HTTP1.1 . Here is a quick recap of the NextProtos config
http1.1 + h2 => works
h2 + http1.1 => curl needs --http1.1 param && readiness are different depending if server is insecure or not => very annoying
h2 => curl/readiness wont work
http1.1 => grpc/UI wont work
nothing => we need to add an env var to disable this, which ends up doing the same as "http1.1 + h2"

Caused by #20579

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 55.14%. Comparing base (11267b9) to head (adb4536).

Files with missing lines	Patch %	Lines
server/server.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #20639   +/-   ##
=======================================
  Coverage   55.13%   55.14%           
=======================================
  Files         324      324           
  Lines       55197    55197           
=======================================
+ Hits        30432    30437    +5     
+ Misses      22148    22137   -11     
- Partials     2617     2623    +6     

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