Update Go to 1.21.1

Apparently something in our dependency tree [no longer builds][1] on Go
1.19. I've gone ahead and bumped to the latest version of Go to fix
this. We could also bump more conveniently to 1.20, but I figured it was
better to stay on the latest; happy to change that if we'd rather just
go to 1.20.8.

[1]: https://github.com/linkerd/dev/actions/runs/6395603721/job/17359636186#step:4:757

Dockerfile

@@ -267,7 +267,7 @@ COPY --link --from=tools-script /bin/* /bin/
 ##
 
 # A Go build environment.
-FROM docker.io/library/golang:1.19.8 as go
+FROM docker.io/library/golang:1.21.1 as go
 RUN --mount=type=cache,from=apt-base,source=/etc/apt,target=/etc/apt,ro \
     --mount=type=cache,from=apt-base,source=/var/cache/apt,target=/var/cache/apt \
     --mount=type=cache,from=apt-base,source=/var/lib/apt/lists,target=/var/lib/apt/lists,ro \