Bump golang from 1.20.7-alpine to 1.21.0-alpine

Bumps golang from 1.20.7-alpine to 1.21.0-alpine.

---
updated-dependencies:
- dependency-name: golang
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Dockerfile

@@ -1,4 +1,4 @@
-FROM --platform=$BUILDPLATFORM golang:1.20.7-alpine AS build
+FROM --platform=$BUILDPLATFORM golang:1.21.0-alpine AS build
 WORKDIR /go/src/github.com/traPtitech/traQ
 
 COPY ./go.* ./