Misc cosmetic cleanups (#12598)

- Remove a few obsolete "allowed error messages" from tests. The
pageserver doesn't emit those messages anymore.

- Remove misplaced and outdated docstring comment from
`test_tenants.py`. A docstring is supposed to be the first thing in a
function, but we had added some code before it. And it was outdated, as
we haven't supported running without safekeepers for a long time.

- Fix misc typos in comments

- Remove obsolete comment about backwards compatibility with safekeepers
without `TIMELINE_STATUS` API. All safekeepers have it by now.

proxy/src/proxy/connect_compute.rs

@@ -110,7 +110,7 @@ where
     debug!(error = ?err, COULD_NOT_CONNECT);
 
     let node_info = if !node_info.cached() || !err.should_retry_wake_compute() {
-        // If we just recieved this from cplane and didn't get it from cache, we shouldn't retry.
+        // If we just received this from cplane and not from the cache, we shouldn't retry.
         // Do not need to retrieve a new node_info, just return the old one.
         if !should_retry(&err, num_retries, compute.retry) {
             Metrics::get().proxy.retries_metric.observe(