[doc] Migrate sinatra example to sinatra 3

Validated it works correctly with a prerelease warbler version on Tomcat 9.

Author: chadlwilson

examples/sinatra/Gemfile

@@ -2,5 +2,6 @@ source 'https://rubygems.org'
 
 gem 'sinatra', '< 4.0'
 gem 'webrick'
-gem "jruby-rack", "~> 1.2"
-gem 'rack', '2.2.14'
+gem "jruby-rack", "~> 1.2.4"
+gem 'rack', '~> 2.2.0'
+gem "jruby-jars", "~> 9.4"

examples/sinatra/Gemfile.lock

@@ -1,18 +1,34 @@
 GEM
   remote: https://rubygems.org/
   specs:
+    base64 (0.3.0)
+    jruby-jars (9.4.13.0)
     jruby-rack (1.2.4)
-    rack (2.2.14)
-    sinatra (1.0)
-      rack (>= 1.0)
+    mustermann (3.0.4)
+      ruby2_keywords (~> 0.0.1)
+    rack (2.2.17)
+    rack-protection (3.2.0)
+      base64 (>= 0.1.0)
+      rack (~> 2.2, >= 2.2.4)
+    ruby2_keywords (0.0.5)
+    sinatra (3.2.0)
+      mustermann (~> 3.0)
+      rack (~> 2.2, >= 2.2.4)
+      rack-protection (= 3.2.0)
+      tilt (~> 2.0)
+    tilt (2.6.1)
     webrick (1.9.1)
 
 PLATFORMS
   java
   x86_64-linux
 
 DEPENDENCIES
-  jruby-rack (~> 1.2)
-  rack (= 2.2.14)
+  jruby-jars (~> 9.4)
+  jruby-rack (~> 1.2.4)
+  rack (~> 2.2.0)
   sinatra (< 4.0)
   webrick
+
+BUNDLED WITH
+   2.6.9