Pin plugins that incorporate breaking changes in 9.0

A number of plugins include breaking changes in 9.x, which, without
restriction, will be included in development `8.x` branches that do
not include a lockfile.

This commit adds these to the Gemfile.template to avoid being introduced
in 8.x development branches

Gemfile.template

@@ -42,3 +42,7 @@ gem "jar-dependencies", "= 0.4.1" # Gem::LoadError with jar-dependencies 0.4.2
 gem "murmurhash3", "= 0.1.6" # Pins until version 0.1.7-java is released
 gem "thwait"
 gem "bigdecimal", "~> 3.1"
+
+# Pin plugins to avoid including those with breaking changes in development builds without a defined lockfile
+gem "logstash-input-beats", "~> 6"
+gem "logstash-output-http", "~> 5"