Simply etcdutl backup command to cleanup v2 related implementation

[ahrtr]

The V2 commands have already been removed in PR 13643 , so we can simplify the the implementation of etcdutl backup command.

[codecov-commenter]

Codecov Report

Merging #13972 (9055296) into main (b00bb53) will decrease coverage by 0.17%.
The diff coverage is 0.00%.

❗ Current head 9055296 differs from pull request most recent head 34b7ba4. Consider uploading reports for the commit 34b7ba4 to get more accurate results

@@            Coverage Diff             @@
##             main   #13972      +/-   ##
==========================================
- Coverage   74.93%   74.76%   -0.18%     
==========================================
  Files         447      447              
  Lines       37166    37152      -14     
==========================================
- Hits        27852    27777      -75     
- Misses       7543     7584      +41     
- Partials     1771     1791      +20     

Flag	Coverage Δ
all	74.76% <0.00%> (-0.18%)	⬇️

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

Impacted Files	Coverage Δ
etcdutl/etcdutl/backup_command.go	10.77% <0.00%> (+0.83%)	⬆️
server/proxy/grpcproxy/register.go	69.76% <0.00%> (-20.94%)	⬇️
client/pkg/v3/fileutil/purge.go	66.03% <0.00%> (-7.55%)	⬇️
client/v3/leasing/util.go	91.66% <0.00%> (-6.67%)	⬇️
client/v3/namespace/watch.go	87.87% <0.00%> (-6.07%)	⬇️
raft/rafttest/node.go	95.00% <0.00%> (-5.00%)	⬇️
client/v3/leasing/txn.go	88.09% <0.00%> (-3.18%)	⬇️
server/etcdserver/cluster_util.go	70.35% <0.00%> (-3.17%)	⬇️
server/etcdserver/api/v3rpc/watch.go	84.89% <0.00%> (-3.03%)	⬇️
client/v3/experimental/recipes/key.go	75.34% <0.00%> (-2.74%)	⬇️
... and 15 more

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[serathius]

cc @ptabor

[ahrtr]

Just rebased this PR.

[ahrtr]

@ptabor @spzala Could you take a look at this PR?

[ptabor]

LGTM - just consider preserving the flag to give user's an actionable advise.

[ptabor]

Sad there was no --v2 test coverage at all :(