Bump alpine from 3.20 to 3.21

Bumps alpine from 3.20 to 3.21.

---
updated-dependencies:
- dependency-name: alpine
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Dockerfile-alpine

@@ -1,7 +1,7 @@
 #
 # Define the base OS image in a single place.
 #
-FROM alpine:3.20 AS base
+FROM alpine:3.21 AS base
 
 #
 # The builder step is where Kea is compiled.