Unpin rbs from the bundled gems auto-update

rbs was pinned in March 2026 while the 4.0 series was breaking CI. Now
that 4.2.0 is in gems/bundled_gems and the tests pass again, the daily
workflow can pick up later rbs releases on its own.

tool/update-bundled_gems.rb

@@ -5,7 +5,7 @@ BEGIN {
   # STDOUT is not usable in inplace edit mode
   output = $-i ? STDOUT : STDERR
   # Gems to skip auto-updating (e.g. when a new major version breaks CI)
-  pinned = %w[rbs]
+  pinned = %w[]
 }
 output = STDERR if ARGF.file == STDIN
 END {