Merge #2200

2200: Switch to ISRG_Root_X1 in front nginx r=mergify[bot] a=packet23

## What type of PR?

bug-fix

## What does this PR do?

### Related issue(s)
Resolves an nginx startup issue when letsencrypt or mail-letsencrypt is enabled.

closes #2199

## Prerequisites
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

- [X] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/workflow.html#changelog) entry file.


Co-authored-by: Sebastian Klemke <3669903+packet23@users.noreply.github.com>
Co-authored-by: Florent Daigniere <nextgens@users.noreply.github.com>

core/nginx/conf/tls.conf

@@ -3,7 +3,7 @@ ssl_certificate_key {{ TLS[1] }};
 {% if TLS_FLAVOR in ['letsencrypt','mail-letsencrypt'] %}
 ssl_certificate {{ TLS[2] }};
 ssl_certificate_key {{ TLS[3] }};
-ssl_trusted_certificate /etc/ssl/certs/ca-cert-DST_Root_CA_X3.pem;
+ssl_trusted_certificate /etc/ssl/certs/ca-cert-ISRG_Root_X1.pem;
 {% endif %}
 ssl_session_timeout 1d;
 ssl_session_tickets off;

towncrier/newsfragments/2199.bugfix

@@ -0,0 +1 @@
+Switch from DST_ROOT_X3 to ISRG_X1 as alpine is not shipping the former anymore