Update alpine Docker tag to v3.21

Author: renovate[bot]

.github/workflows/ci.yml

@@ -82,7 +82,7 @@ jobs:
         operating_system:
           - flavor: "alpine"
             # renovate: datasource=docker depName=alpine versioning=docker
-            OS_VERSION: "3.20"
+            OS_VERSION: "3.21"
           - flavor: "bookworm"
         pg_target:
           - "12"

Dockerfile.alpine

@@ -1,7 +1,7 @@
 ARG PGTARGET=17
 
 # renovate: datasource=docker depName=alpine versioning=loose
-ARG ALPINE_VERSION=3.20
+ARG ALPINE_VERSION=3.21
 
 ### Things we need in all build containers
 FROM alpine:${ALPINE_VERSION} AS base-build