Bump rake dev dependency to 13

Quiet a GitHub alert about GHSA-jppv-gw3r-w3q8

Author: HarlemSquirrel

lib/net/ldap/connection.rb

@@ -181,7 +181,7 @@ def setup_encryption(args, timeout=nil)
   # have to call it, but perhaps it will come in handy someday.
   #++
   def close
-    return if @conn.nil?
+    return if !defined?(@conn) || @conn.nil?
     @conn.close
     @conn = nil
   end

net-ldap.gemspec

@@ -30,7 +30,7 @@ the most recent LDAP RFCs (4510-4519, plutions of 4520-4532).}
   s.summary = %q{Net::LDAP for Ruby (also called net-ldap) implements client access for the Lightweight Directory Access Protocol (LDAP), an IETF standard protocol for accessing distributed directory services}
 
   s.add_development_dependency("flexmock", "~> 1.3")
-  s.add_development_dependency("rake", "~> 10.0")
+  s.add_development_dependency("rake", "~> 13.0")
   s.add_development_dependency("rubocop", "~> 0.49.0")
   s.add_development_dependency("test-unit")
   s.add_development_dependency("byebug")