Changed: .ruby-version back to 9.1.12.0

.ruby-version is used to select the external jruby
(for package building + acceptance tests on infra)

reverts the upgraded JRuby version from elastic#11647

Fixes elastic#11663

Author: kares

.ruby-version

@@ -1 +1 @@
-jruby-9.2.11.0
+jruby-9.1.12.0