Update warbler fixture to 2.1.0 for JRuby 10.1

warbler 2.0.5 (from the github pull/557 pin) does not work on JRuby
10.1: it cannot require the now-bundled ostruct, and even after that
hurdle Monitor#synchronize raises ArgumentError under the warble jar
build. warbler 2.1.0 explicitly supports JRuby 10.1 and pulls ostruct
in itself, so switch the fixture over to the gem release.

Author: hsbt

spec/realworld/fixtures/warbler/Gemfile

@@ -4,4 +4,4 @@ source "https://rubygems.org"
 
 gem "demo", path: "./demo"
 gem "jruby-jars", "~> 10.0"
-gem "warbler", github: "https://github.com/jruby/warbler/pull/557"
+gem "warbler", "~> 2.1"

spec/realworld/fixtures/warbler/Gemfile.lock

@@ -1,15 +1,3 @@
-GIT
-  remote: https://github.com/jruby/warbler.git
-  revision: 3a3a89e9a055ab1badb4e6fee860e8617b4acfe1
-  ref: refs/pull/557/head
-  specs:
-    warbler (2.0.5)
-      jruby-jars (>= 9.0.0)
-      jruby-rack (>= 1.1.1, < 1.3)
-      rake (>= 13.0.3)
-      rexml (~> 3.0)
-      rubyzip (>= 1.0.0)
-
 PATH
   remote: demo
   specs:
@@ -19,10 +7,18 @@ GEM
   remote: https://rubygems.org/
   specs:
     jruby-jars (10.0.0.1)
-    jruby-rack (1.2.2)
+    jruby-rack (1.2.7)
+    ostruct (0.6.3)
     rake (13.3.0)
     rexml (3.4.2)
-    rubyzip (2.4.1)
+    rubyzip (3.3.0)
+    warbler (2.1.0)
+      jruby-jars (>= 9.4, < 10.1)
+      jruby-rack (>= 1.2.3, < 1.3)
+      ostruct (~> 0.6.2)
+      rake (~> 13.0, >= 13.0.3)
+      rexml (~> 3.0)
+      rubyzip (>= 3.0.0)
 
 PLATFORMS
   arm64-darwin
@@ -33,7 +29,7 @@ PLATFORMS
 DEPENDENCIES
   demo!
   jruby-jars (~> 10.0)
-  warbler!
+  warbler (~> 2.1)
 
 BUNDLED WITH
   4.1.0.dev