Update docker.io/library/golang Docker tag to v1.21

Generated by renovateBot

Author: renovate[bot]

e2e/Dockerfile

@@ -1,7 +1,7 @@
 # SPDX-FileCopyrightText: 2023 The Pion community <https://pion.ly>
 # SPDX-License-Identifier: MIT
 
-FROM docker.io/library/golang:1.20-bullseye
+FROM docker.io/library/golang:1.21-bullseye
 
 COPY . /go/src/github.com/pion/dtls
 WORKDIR /go/src/github.com/pion/dtls/e2e