*: document deprecated ca-file flags (since v2.1)

[gyuho]

ca-file flags were deprecated in v2.1 (3 years ago).
Should be safe to deprecate.
Checked Kubernetes upstream not using these flags either.

Fix #9432.

[codecov-io]

Codecov Report

Merging #9470 into master will increase coverage by 0.23%.
The diff coverage is 93.75%.

@@            Coverage Diff            @@
##           master   #9470      +/-   ##
=========================================
+ Coverage   72.47%   72.7%   +0.23%     
=========================================
  Files         364     364              
  Lines       30890   30883       -7     
=========================================
+ Hits        22389   22455      +66     
+ Misses       6872    6809      -63     
+ Partials     1629    1619      -10

Impacted Files	Coverage Δ
etcdmain/config.go	81.71% <ø> (-0.21%)	⬇️
clientv3/yaml/config.go	86.66% <ø> (+5.41%)	⬆️
embed/config.go	60.71% <0%> (-0.14%)	⬇️
pkg/transport/listener.go	82.03% <100%> (-0.28%)	⬇️
etcdctl/ctlv3/command/global.go	60.6% <100%> (ø)	⬆️
etcdmain/grpc_proxy.go	63.42% <100%> (ø)	⬆️
etcdctl/ctlv3/ctl.go	100% <100%> (ø)	⬆️
etcdctl/ctlv2/command/util.go	44.57% <100%> (ø)	⬆️
pkg/transport/timeout_conn.go	80% <0%> (-20%)	⬇️
proxy/grpcproxy/register.go	69.44% <0%> (-16.67%)	⬇️
... and 23 more

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