fix: Correct base image in 1.21/main

[mahendrapaipuri]

Hello,

Looking into tag 1.21-main, I found that the base image is incorrect. Tag 1.21-main is using 1.19-base as base image and hence, go version inside the container is still 1.19.

This can be verified as follows:

docker run -v $PWD:/app -it --rm --entrypoint /bin/bash quay.io/prometheus/golang-builder:1.21-main
root@bb3024ca8c3f:/app# go version
go version go1.19.12 linux/amd64
root@bb3024ca8c3f:/app# which go
/usr/local/go/bin/go

This PR corrects the base image tag to use 1.21-base.

[mahendrapaipuri]

@SuperQ can you please take a look into it? We are currently blocked by go version in the image. Cheers!

[SuperQ]

Very strange, I wonder if the version upgrade handler has a bug.