pageserver: increase DEFAULT_MAX_WALRECEIVER_LSN_WAL_LAG (#6970)

## Problem

At high ingest rates, pageservers spuriously disconnect from safekeepers
because stats updates don't come in frequently enough to keep the
broker/safekeeper LSN delta under the wal lag limit.

## Summary of changes

- Increase DEFAULT_MAX_WALRECEIVER_LSN_WAL_LAG from 10MiB to 1GiB. This
should be enough for realistic per-timeline throughputs.

test_runner/regress/test_tenant_conf.py

@@ -270,7 +270,7 @@ eviction_policy = { "kind" = "LayerAccessThreshold", period = "20s", threshold =
         "period": "20s",
         "threshold": "23h",
     }
-    assert final_effective_config["max_lsn_wal_lag"] == 10 * 1024 * 1024
+    assert final_effective_config["max_lsn_wal_lag"] == 1024 * 1024 * 1024
 
     # restart the pageserver and ensure that the config is still correct
     env.pageserver.stop()