pageserver: downgrade stale generation messages to INFO (#8256)

## Problem

When generations were new, these messages were an important way of
noticing if something unexpected was going on. We found some real issues
when investigating tests that unexpectedly tripped them.

At time has gone on, this code is now pretty battle-tested, and as we do
more live migrations etc, it's fairly normal to see the occasional
message from a node with a stale generation.

At this point the cognitive load on developers to selectively allow-list
these logs outweighs the benefit of having them at warn severity.

Closes: https://github.com/neondatabase/neon/issues/8080

## Summary of changes

- Downgrade "Dropped remote consistent LSN updates" and "Dropping stale
deletions" messages to INFO
- Remove all the allow-list entries for these logs.

test_runner/performance/test_storage_controller_scale.py

@@ -132,14 +132,6 @@ def test_storage_controller_many_tenants(
     )
 
     for ps in env.pageservers:
-        # This can happen because when we do a loop over all pageservers and mark them offline/active,
-        # reconcilers might get cancelled, and the next reconcile can follow a not-so-elegant path of
-        # bumping generation before other attachments are detached.
-        #
-        # We could clean this up by making reconcilers respect the .observed of their predecessor, if
-        # we spawn with a wait for the predecessor.
-        ps.allowed_errors.append(".*Dropped remote consistent LSN updates.*")
-
         # Storage controller is allowed to drop pageserver requests when the cancellation token
         # for a Reconciler fires.
         ps.allowed_errors.append(".*request was dropped before completing.*")