server: Handle cluster version equal downgrade version

[serathius]

cc @ahrtr @spzala @tjungblu

[codecov-commenter]

Codecov Report

Merging #14588 (2b178fd) into main (a861bfe) will increase coverage by 0.01%.
The diff coverage is 92.59%.

@@            Coverage Diff             @@
##             main   #14588      +/-   ##
==========================================
+ Coverage   75.41%   75.42%   +0.01%     
==========================================
  Files         457      457              
  Lines       37213    37223      +10     
==========================================
+ Hits        28063    28077      +14     
+ Misses       7380     7378       -2     
+ Partials     1770     1768       -2     

Flag	Coverage Δ
all	75.42% <92.59%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
server/etcdserver/server.go	84.81% <33.33%> (-0.21%)	⬇️
server/etcdserver/version/monitor.go	96.69% <100.00%> (+0.23%)	⬆️
server/etcdserver/txn/util.go	75.47% <0.00%> (-3.78%)	⬇️
api/etcdserverpb/raft_internal_stringer.go	76.78% <0.00%> (-3.58%)	⬇️
client/v3/leasing/txn.go	88.09% <0.00%> (-3.18%)	⬇️
server/etcdserver/cluster_util.go	70.35% <0.00%> (-3.17%)	⬇️
client/v3/experimental/recipes/key.go	75.34% <0.00%> (-2.74%)	⬇️
pkg/adt/interval_tree.go	85.96% <0.00%> (-1.26%)	⬇️
server/proxy/grpcproxy/watch.go	92.48% <0.00%> (-1.16%)	⬇️
server/auth/store.go	84.21% <0.00%> (-1.16%)	⬇️
... and 20 more

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[ahrtr]

LGTM