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/regress/test_layers_from_future.py

@@ -39,9 +39,6 @@ def test_issue_5878(neon_env_builder: NeonEnvBuilder):
 
     env = neon_env_builder.init_configs()
     env.start()
-    env.pageserver.allowed_errors.extend(
-        [".*Dropped remote consistent LSN updates.*", ".*Dropping stale deletions.*"]
-    )
 
     ps_http = env.pageserver.http_client()