Fix duplicate default merge array values

What does this merge request do and why?

In main branch, if I set a GDK config with gdk config set <key> <value>, this line gets added, even if it already exists.

Questions

• Should it get added if it doesn't exist but I've specified a user-defined array value? In this MR, it still gets added. That seems a bit user-hostile.
• Furthermore, what if you set an Array config to empty, because you just don't want the default?

A way to resolve both questions: If a value is so fundamentally important, maybe it should be implied, and not added to gdk.yml.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Before

➜  gdk git:(main) gdk config get gdk.update_hooks.before
support/exec-cd gitlab bin/spring stop || true
support/exec-cd gitlab bin/spring stop || true
rake gitlab:truncate_logs
➜  gdk git:(main) gdk config get gdk.update_hooks.before
support/exec-cd gitlab bin/spring stop || true
support/exec-cd gitlab bin/spring stop || true
rake gitlab:truncate_logs
➜  gdk git:(main) gdk config set webpack.incremental true
✅️ 'webpack.incremental' is now set to 'true' (previously 'false').
ℹ️  A backup of 'gdk.yml' has been made at '.backups/gdk.yml.20230413004218'.
ℹ️  Don't forget to run 'gdk reconfigure'.
➜  gdk git:(main) gdk config get gdk.update_hooks.before                            
support/exec-cd gitlab bin/spring stop || true
support/exec-cd gitlab bin/spring stop || true
support/exec-cd gitlab bin/spring stop || true
rake gitlab:truncate_logs
➜  gdk git:(main) gdk config set webpack.incremental false
✅️ 'webpack.incremental' is now set to 'false' (previously 'true').
ℹ️  A backup of 'gdk.yml' has been made at '.backups/gdk.yml.20230413004230'.
ℹ️  Don't forget to run 'gdk reconfigure'.
➜  gdk git:(main) gdk config get gdk.update_hooks.before  
support/exec-cd gitlab bin/spring stop || true
support/exec-cd gitlab bin/spring stop || true
support/exec-cd gitlab bin/spring stop || true
support/exec-cd gitlab bin/spring stop || true
rake gitlab:truncate_logs

After

➜  gdk git:(main) gco mk/fix-duplicate-default-merge-arrays 
Switched to branch 'mk/fix-duplicate-default-merge-arrays'
Your branch is up to date with 'origin/mk/fix-duplicate-default-merge-arrays'.
➜  gdk git:(mk/fix-duplicate-default-merge-arrays) echo "Manually deleting the excess array values..."  
Manually deleting the excess array values...
➜  gdk git:(mk/fix-duplicate-default-merge-arrays) gdk config get gdk.update_hooks.before
support/exec-cd gitlab bin/spring stop || true
rake gitlab:truncate_logs
➜  gdk git:(mk/fix-duplicate-default-merge-arrays) gdk config set webpack.incremental true            
✅️ 'webpack.incremental' is now set to 'true' (previously 'false').
ℹ️  A backup of 'gdk.yml' has been made at '.backups/gdk.yml.20230413004351'.
ℹ️  Don't forget to run 'gdk reconfigure'.
➜  gdk git:(mk/fix-duplicate-default-merge-arrays) gdk config get gdk.update_hooks.before 
support/exec-cd gitlab bin/spring stop || true
rake gitlab:truncate_logs
➜  gdk git:(mk/fix-duplicate-default-merge-arrays) gdk config set webpack.incremental false           
✅️ 'webpack.incremental' is now set to 'false' (previously 'true').
ℹ️  A backup of 'gdk.yml' has been made at '.backups/gdk.yml.20230413004359'.
ℹ️  Don't forget to run 'gdk reconfigure'.
➜  gdk git:(mk/fix-duplicate-default-merge-arrays) gdk config get gdk.update_hooks.before  
support/exec-cd gitlab bin/spring stop || true
rake gitlab:truncate_logs

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• [-] Tests added for new functionality. If not, please raise an issue to follow-up.
• [-] Documentation added/updated, if needed.
• [-] Announcement added, if change is notable.
• [-] gdk doctor test added, if needed.
• [-] Add the ~highlight label if this MR should be included in the CHANGELOG.md.