Remove string dependent error handling in watch and auth

[mitake]

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

If we cannot finish #15253 before 3.6, it's better to drop the string dependent error handling from the main branch.
cc @ahrtr

[mitake]

It's better to avoid introducing the string dependent behavior in the main branch and 3.6, so reverting it now is safer.
Once people can allocate time, discussing again about #15253 and introducing the flag would be good.

[ahrtr]

Thanks @mitake

To be clearer, this PR is reverting #14935 and partially revert #14322 .

• Since the server side change has already been backported to 3.5 (related to populating different string for cancelReason based on different errors), and it's OK to keep that part unchanged.
• Please also remove AuthTokenTTL from tests/framework/integration/cluster.go.

Regarding the next step, we have multiple choice,

• Keep it as it'is. In other words, do nothing (probably we need to update the doc). If applications see any issue, they just need to recreate the client.
  • Good side:
    • If Auth isn't enabled, then the issue watch stream return "permission denied" if token expired #12385 doesn't exist. So it has no impact on K8s at all.
  • Bad side
    • It isn't reasonable to request applications to recreate the etcd client
• Adopt solution something like etcdserverpb: add "retry" flag to WatchResponse #15253
  • Good side
    • It can fix the issue completely
  • Bad side
    • It adds new field, and complicate the use case of downgrade (e.g. 3.5 -> 3.4)
    • It isn't flexible (as compared to the adding of reasonCode), but we don't see other valid cases in which the client sides will use the reasonCode.

Given our bandwidth and it isn't a critical issue (not even a major issue), prefer to the first solution (keep it as it's)?
@mitake @serathius @lavacat @liggitt

EDIT (Feb 8, 2024): When using TLS CN based auth, this issue (#12385) can't be reproduced either.

So workaround for the issue:

• Try to use TLS CN based auth
• If the client is still using username:password, then re-create the client when running into the issue.

[ahrtr]

• Please also remove AuthTokenTTL from tests/framework/integration/cluster.go.

Let me take care of this separately in a followup PR.

[ahrtr]

lgtm

cc. @serathius