get_lsn_by_timestamp: clamp commit_lsn to be >= min_lsn (#7488)

There was an edge case where
`get_lsn_by_timestamp`/`find_lsn_for_timestamp` could have returned an
lsn that is before the limits we enforce: when we did find SLRU entries
with timestamps before the one we search for.

The API contract of `get_lsn_by_timestamp` is to not return something
before the anchestor lsn.

cc https://neondb.slack.com/archives/C03F5SM1N02/p1713871064147029

pageserver/src/pgdatadir_mapping.rs

@@ -448,6 +448,11 @@ impl Timeline {
         // include physical changes from later commits that will be marked
         // as aborted, and will need to be vacuumed away.
         let commit_lsn = Lsn((low - 1) * 8);
+        // This maxing operation is for the edge case that the search above did
+        // set found_smaller to true but it never increased the lsn. Then, low
+        // is still the old min_lsn the subtraction above could possibly give a value
+        // below the anchestor_lsn.
+        let commit_lsn = commit_lsn.max(min_lsn);
         match (found_smaller, found_larger) {
             (false, false) => {
                 // This can happen if no commit records have been processed yet, e.g.