tests: Fix cluster version and downgrade request timeout

[serathius]

Returning nil means that raft.Trigger was not called, causing member to
wait infinitly for response for response on raft request.

cc @ptabor @ahrtr @spzala

[codecov-commenter]

Codecov Report

Merging #13694 (d261ffc) into main (bcadd03) will increase coverage by 0.06%.
The diff coverage is 42.85%.

❗ Current head d261ffc differs from pull request most recent head b599a33. Consider uploading reports for the commit b599a33 to get more accurate results

@@            Coverage Diff             @@
##             main   #13694      +/-   ##
==========================================
+ Coverage   72.51%   72.57%   +0.06%     
==========================================
  Files         465      465              
  Lines       37865    37872       +7     
==========================================
+ Hits        27457    27485      +28     
+ Misses       8620     8604      -16     
+ Partials     1788     1783       -5     

Flag	Coverage Δ
all	72.57% <42.85%> (+0.06%)	⬆️

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

Impacted Files	Coverage Δ
server/etcdserver/apply.go	89.32% <33.33%> (ø)
server/storage/mvcc/index.go	90.25% <33.33%> (-2.41%)	⬇️
server/auth/simple_token.go	88.46% <50.00%> (+7.35%)	⬆️
server/etcdserver/version/version.go	44.82% <100.00%> (ø)
client/v3/namespace/watch.go	87.87% <0.00%> (-6.07%)	⬇️
client/v3/leasing/cache.go	87.77% <0.00%> (-3.89%)	⬇️
server/etcdserver/api/v3rpc/member.go	93.54% <0.00%> (-3.23%)	⬇️
client/v3/leasing/kv.go	89.03% <0.00%> (-1.67%)	⬇️
server/lease/leasehttp/http.go	64.23% <0.00%> (-1.46%)	⬇️
etcdctl/ctlv3/command/printer_simple.go	76.92% <0.00%> (-1.29%)	⬇️
... and 17 more

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[ahrtr]

LGTM overall.