server/etcdmain: officially supports "arm64"

[gyuho]

• Integration and end-to-end tests are passing (ref. github action: add ARM64 test #12928)
• I've been extensively testing etcd on arm64 and so far, haven't faced any critical issue.
• I am proposing to have tiered support system supported-platform: define platform support tiers based on CI website#273, listing myself as a maintainer. arm64 will be tier-2 for now, until we run more functional tests. But, it should be now marked as officially supported since it's the only platform other than amd64, that runs and passes all integration tests.

[codecov-commenter]

Codecov Report

Merging #12929 (5269b1e) into master (aeb9b5f) will decrease coverage by 0.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #12929      +/-   ##
==========================================
- Coverage   73.23%   73.01%   -0.22%     
==========================================
  Files         430      430              
  Lines       34182    34185       +3     
==========================================
- Hits        25032    24959      -73     
- Misses       7218     7288      +70     
- Partials     1932     1938       +6     

Flag	Coverage Δ
all	73.01% <100.00%> (-0.22%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/etcdmain/etcd.go	43.91% <100.00%> (+0.57%)	⬆️
server/etcdserver/api/rafthttp/peer.go	77.41% <0.00%> (-18.07%)	⬇️
server/etcdserver/api/rafthttp/peer_status.go	89.28% <0.00%> (-10.72%)	⬇️
client/pkg/v3/tlsutil/tlsutil.go	83.33% <0.00%> (-8.34%)	⬇️
server/etcdserver/api/v3rpc/lease.go	74.64% <0.00%> (-5.64%)	⬇️
server/proxy/grpcproxy/watch.go	93.86% <0.00%> (-3.69%)	⬇️
server/etcdserver/api/v3rpc/util.go	67.85% <0.00%> (-3.58%)	⬇️
client/pkg/v3/testutil/recorder.go	76.27% <0.00%> (-3.39%)	⬇️
client/v3/experimental/recipes/key.go	72.13% <0.00%> (-3.28%)	⬇️
server/etcdserver/api/v3rpc/watch.go	84.67% <0.00%> (-2.92%)	⬇️
... and 16 more

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[gyuho]

/cc @mkumatag @spzala