Bump TargetChefVersion to 14.15.6

Summary:
This enables the following cops:

- Chef/Deprecations/DeprecatedShelloutMethods
- Chef/Deprecations/WindowsVersionHelpers
- Chef/Modernize/UnnecessaryDependsChef14
- Chef/Modernize/CronDFileOrTemplate
- Chef/Modernize/CronManageResource
- Chef/Modernize/ExecuteScExe
- Chef/Modernize/ExecuteSysctl
- Chef/Modernize/ExecuteTzUtil
- Chef/Modernize/MacOsXUserdefaults
- Chef/Modernize/OpensslRsaKeyResource
- Chef/Modernize/OpensslX509Resource
- Chef/Modernize/PowershellInstallWindowsFeature
- Chef/Modernize/WindowsScResource
- Chef/Modernize/SysctlParamResource

Differential Revision: D45967167

fbshipit-source-id: 9491a32ca63b6256f4cb9c54e805a092e110f00a

Author: dafyddcrosby

.cookstyle.yml

@@ -5,7 +5,7 @@ AllCops:
   # new cops and make upgrades easier (ie not dealing with a bunch of noise
   # each Cookstyle update)
   DisabledByDefault: true
-  TargetChefVersion: 13.10.0
+  TargetChefVersion: 14.15.6
 
 Chef/Correctness/BlockGuardWithOnlyString:
   Enabled: true

cookbooks/fb_sysstat/recipes/cron.rb

@@ -56,6 +56,6 @@
 
 # the sa[12] commands here trample on those defined in the
 # sysstat_accounting_[12] jobs
-file '/etc/cron.d/sysstat' do
+file '/etc/cron.d/sysstat' do # rubocop:disable Chef/Modernize/CronDFileOrTemplate
   action :delete
 end