Bump alpine from 3.20 to 3.21 in /core

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] <support@github.com>

core/Dockerfile

@@ -1,6 +1,6 @@
 # syntax = docker/dockerfile:1.3
 # Use 3.15 for Core since it has Python 3.9
-FROM alpine:3.20
+FROM alpine:3.21
 
 # Add requirements file.
 COPY requirements.txt /tmp/

core/Dockerfile.dev

@@ -1,6 +1,6 @@
 # syntax = docker/dockerfile:1.3
 # Use 3.15 for Core since it has Python 3.9
-FROM alpine:3.20
+FROM alpine:3.21
 
 # Set the commits that we are building.
 ARG CORE_REF