unix/linux: update to Linux kernel 6.4 and Go 1.21rc2

Also fix the glibc 2.37 release date in a comment.

Change-Id: Ibd972a2846f0085bacf67c847ce4726bf130d5ba
Reviewed-on: https://go-review.googlesource.com/c/sys/+/506017
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Carlos Amedee <carlos@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

Author: tklauser

unix/linux/Dockerfile

@@ -15,15 +15,15 @@ RUN apt-get update && apt-get install -y  --no-install-recommends \
 # Get the git sources. If not cached, this takes O(5 minutes).
 WORKDIR /git
 RUN git config --global advice.detachedHead false
-# Linux Kernel: Released 23 Apr 2023
-RUN git clone --branch v6.3 --depth 1 https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux
-# GNU C library: Released 1 Feb 2022
+# Linux Kernel: Released 25 June 2023
+RUN git clone --branch v6.4 --depth 1 https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux
+# GNU C library: Released 1 Feb 2023
 RUN git clone --branch release/2.37/master --depth 1 https://sourceware.org/git/glibc.git
 
 # Get Go
-ENV GOLANG_VERSION 1.20.5
+ENV GOLANG_VERSION 1.21rc2
 ENV GOLANG_DOWNLOAD_URL https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
-ENV GOLANG_DOWNLOAD_SHA256 d7ec48cde0d3d2be2c69203bc3e0a44de8660b9c09a6e85c4732a3f7dc442612
+ENV GOLANG_DOWNLOAD_SHA256 8fe90332727c606019e80a7368e23f5e65ad59520e45ee4010692f15572e45c6
 
 RUN curl -fsSL "$GOLANG_DOWNLOAD_URL" -o golang.tar.gz \
     && echo "$GOLANG_DOWNLOAD_SHA256  golang.tar.gz" | sha256sum -c - \