[IT-3165] remove service user (#973)

We have switch all repos that use this service user from travis to
github action (OIDC) therefore we should remove the service user.
The only difference in the updated template is the removal of the
service user.

sceptre/admincentral/config/prod/bootstrap.yaml

@@ -1,4 +1,4 @@
 template:
   type: http
-  url: https://{{stack_group_config.admincentral_cf_bucket}}.s3.amazonaws.com/aws-infra/v0.3.0/bootstrap.yaml
+  url: https://{{stack_group_config.admincentral_cf_bucket}}.s3.amazonaws.com/aws-infra/v0.7.6/bootstrap.yaml
 stack_name: bootstrap