Fix for solid queue error when db pool count mismatched

Solid Queue is configured to have a pool of 4. A change was recently
added to error when it detects a mismatch in the DB pool count.

rails/solid_queue#457

This change updates config/database.yml to match the Solid Queue
default.

config/database.yml

@@ -7,7 +7,7 @@
 default: &default
   encoding: utf8
   adapter: sqlite3
-  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 3 } %>
+  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 4 } %>
   timeout: 5000
   extensions:
     - SqliteExt::Ulid