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] <support@github.com>

Dockerfile.alpine

@@ -25,7 +25,7 @@ RUN set -xe && \
     export COMMIT_SHA=${COMMIT_SHA:-$(git describe --always --abbrev=8 --dirty)} && \
     go build $(if [ -n "${FEATURES}" ]; then echo "-tags ${FEATURES}"; fi) -trimpath -ldflags "-s -w -X github.com/drakkan/sftpgo/v2/internal/version.commit=${COMMIT_SHA} -X github.com/drakkan/sftpgo/v2/internal/version.date=`date -u +%FT%TZ`" -v -o sftpgo
 
-FROM alpine:3.20
+FROM alpine:3.21
 
 # Set to "true" to install jq and the optional git and rsync dependencies
 ARG INSTALL_OPTIONAL_PACKAGES=false