build(deps): 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]>

Author: dependabot[bot]

Dockerfile

@@ -16,7 +16,7 @@ COPY . .
 RUN make all
 
 ## Runtime image ##
-FROM alpine:3.20 as run
+FROM alpine:3.21 as run
 
 COPY --from=build /usr/src/zgrab2/cmd/zgrab2/zgrab2 /usr/bin/zgrab2