server: background tasks incorrectly hang off the server start context

[knz]

(found while working on #90384)

During server startup, in (*Server).PreStart() we have 2 contexts of interest:

• ctx is bound to the top-level CLI command, and is tracing the initialization of the server. It has a tracing span attached.
• workersCtx is a background context with a few log tags but no tracing span.

The intention is to use workersCtx for all async tasks spawned from there.

However, as of this writing the following do not use workersCtx:

• s.startSystemLogsGC()
• startPurgeOldSessions()
• s.ctSender.Run() (this one likely not affected, because it doesn't generate trace events)
• s.startAttemptUpgrade() -- this is likely intentional: we probably want the ability to inspect a trace if the auto-upgrade fails. However, this intention should be marked with a comment.
• s.node.tenantSettingsWatcher.Start()
• s.kvProber.Start()
• publishPendingLossOfQuorumRecoveryEvents()

Jira issue: CRDB-20832

[knz]

Andrei says this doesn't leak memory because the RunAsyncTask calls under these functions already detach the trace.

[knz]

The other reason to use workersCtx is that ctx has a log tag server start in it. It's not adequate to keep it for background tasks.