Restore cgi pin (elastic#17774)

This commit restores the cgi pin as the default in jruby is still 0.3.6.

Author: donoghuc

Gemfile.template

@@ -42,4 +42,5 @@ gem "murmurhash3", "= 0.1.6" # Pins until version 0.1.7-java is released
 gem "date", "= 3.3.3"
 gem "thwait"
 gem "bigdecimal", "~> 3.1"
+gem "cgi", "~> 0.3.7" # Pins until a new jruby version with updated cgi is released (https://github.com/jruby/jruby/issues/8919)
 gem "jar-dependencies", "= 0.5.4" # Pin to avoid conflict with default