manual rebase of #25554

[jbardin]

Rebased commit from #25554. This updates the client and removes a lot of old dependencies. There are newer module-compatible releases of the client since the commit being pulled here was released, but there appears to be some breaking changes between the versions, so I left the commit at the specified has rather than attempting to update further.

---

etcd rewrote its import path from coreos/etcd to go.etcd.io/etcd.
Changed the imports path in this commit, which also updates the code
version.

This lets us remove the github.com/ugorji/go/codec dependency, which
was pinned to a fairly old version. The net change is a loss of 30,000
lines of code in the vendor directory. (I first noticed this problem
because the outdated go/codec dependency was causing a dependency
failure when I tried to put Terraform and another project in the same
vendor directory.)

Note the version shows up funkily in go.mod, but I verified
visually it's the same commit as the "release-3.4" tag in
github.com/coreos/etcd. The etcd team plans to fix the release version
tagging in v3.5, which should be released soon.

[mildwonkey]

I like your decision to pin to the version from the original pr 👍

[jbardin]

ACC tests can be checked now that #28128 is merged.

--- PASS: TestBackend_impl (0.00s)
--- PASS: TestBackend (7.64s)
--- PASS: TestBackend_lockDisabled (0.13s)
--- PASS: TestRemoteClient_impl (0.00s)
--- PASS: TestRemoteClient (0.32s)
--- PASS: TestEtcdv3_stateLock (2.58s)
--- PASS: TestEtcdv3_destroyLock (0.45s)

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