Cleanup .gitignore based on latest manifest entries

There have been new repos added in stx/manifest that
were not added to the .gitignore in this repo.
This means that there would have been untracked changes
when running:

repo status
 --     stx/cert-manager-armada-app/
 --     stx/kernel/
 --     stx/nginx-ingress-controller-armada-app/

Pertains to:
https://review.opendev.org/#/c/718787/
https://review.opendev.org/#/c/716117/

Story: 2007360
Task: 39426
Change-Id: Ib55639735b181625fb4156f55d4b7814912f9132
Signed-off-by: albailey <Al.Bailey@windriver.com>

stx/.gitignore

@@ -7,6 +7,7 @@
 *~
 installer-prebuilt
 /ansible-playbooks
+/cert-manager-armada-app
 /clients
 /compile
 /config
@@ -22,10 +23,12 @@ installer-prebuilt
 /ha
 /helm-charts
 /integ
+/kernel
 /metal
 /monitor-armada-app
 /monitoring
 /nfv
+/nginx-ingress-controller-armada-app
 /oidc-auth-armada-app
 /openstack-armada-app
 /platform-armada-app