Bump alpine from 3.20 to 3.21 (#412)

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]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Dockerfile

@@ -23,7 +23,7 @@ RUN CGO_ENABLED=1 GOOS=linux GOARCH=$TARGETARCH \
     CGO_LDFLAGS="-fuse-ld=mold -lsasl2" \
     go build -tags musl -ldflags "-w -s" .
 
-FROM alpine:3.20
+FROM alpine:3.21
 
 WORKDIR /flyway