Replace File.exists? with File.exist?

File.exists? was removed in Ruby 4.0 / JRuby 10.1, causing
JDBC driver loading to fail with NoMethodError.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: yahonda

lib/plsql/jdbc_connection.rb

@@ -25,7 +25,7 @@
     ["./lib"].concat($LOAD_PATH).concat(env_path).detect do |dir|
       # check any compatible JDBC driver in the priority order
       ojdbc_jars.any? do |ojdbc_jar|
-        if File.exists?(file_path = File.join(dir, ojdbc_jar))
+        if File.exist?(file_path = File.join(dir, ojdbc_jar))
           require file_path
           true
         end