server: rm gossip usage from node/store/cluster ID allocation

In 20.2, with cockroachdb#52526, we introduced the join RPC for new nodes in the
system to be handed a pre-allocated node ID, a pre-allocated store ID,
and be informed by the existing cluster what the cluster ID was.
In 20.1 and earlier this functionality was provided by our use of
gossip, and by constructing hollowed out servers (without node/store
IDs) and allocating an ID at the joining node. This required an
awkward dance around needing KV to be up in order to allocate the right
IDs, but not having an ID for the server process itself.

We retained the deprecated gossip codepaths in 20.2 in order to maintain
compatibility with 20.1. Now that 20.2 is cut however, in 21.1 code we
can always assume that we're talking to nodes that are at least 20.2,
and therefore nodes that are able to use the join RPC correctly. This
lets us strip out our usage of gossip for ID allocation, which in turn
paves the way for a few other simplifications.

We also delete the mixed-version/join-init roachtest, as it was only
ever relevant for the 20.1/20.2 cycle. The current structure around
joining nodes being allocated/informed of the right IDs has adequate
coverage with TestClusterConnectivity.

Release note: None

pkg/cmd/roachtest/BUILD.bazel

@@ -61,7 +61,6 @@ go_library(
         "many_splits.go",
         "mixed_version_decommission.go",
         "mixed_version_jobs.go",
-        "mixed_version_join_init.go",
         "mixed_version_schemachange.go",
         "multitenant.go",
         "namespace_upgrade.go",

pkg/cmd/roachtest/registry.go

@@ -49,7 +49,6 @@ func registerTests(r *testRegistry) {
 	registerInterleaved(r)
 	registerJepsen(r)
 	registerJobsMixedVersions(r)
-	registerJoinInitMixed(r)
 	registerKV(r)
 	registerKVContention(r)
 	registerKVQuiescenceDead(r)