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

@@ -1,4 +1,4 @@
-FROM alpine:3.20 AS builder
+FROM alpine:3.21 AS builder
 
 LABEL maintainer="InspIRCd Team <noreply@inspircd.org>"
 
@@ -34,7 +34,7 @@ RUN make -j`getconf _NPROCESSORS_ONLN` install
 RUN rm -rf /inspircd/conf/*
 
 # Stage 1: Create optimized runtime container
-FROM alpine:3.20
+FROM alpine:3.21
 
 ARG RUN_DEPENDENCIES=