Fix templates to address gomplate-4.x changes

Summary

Following up on gitlab-org/build/CNG!2266 (comment 2396238638)

we need to address "breaking changes" introduced by gomplate-4.x:

We have one item for slice without coll.Slice:

charts/gitlab/charts/webservice/templates/_helpers.tpl:85:{%- $supported_providers := slice "AWS" "AzureRM" "Google" -%}

There's a single item for conv.ToInt64 (within "Functions returning errors instead of quietly returning 0 on invalid input ")

charts/gitlab/charts/gitaly/templates/_configmap_spec.yaml:71: {% $index := index (.Env.HOSTNAME | strings.Split "-" | coll.Reverse) 0 | conv.ToInt64 %}

More details in https://github.com/hairyhenderson/gomplate/releases/tag/v4.0.0