Have khan-dotfiles install go 1.21 now. (#90)

## Summary:
This is step 2(d)(i) from
https://khanacademy.atlassian.net/wiki/spaces/ENG/pages/949387416/Go+version+upgrades

I updated the kabuild Dockerfile too, even though it's not used
anymore.  We should just delete the entire containers/ directory.

Issue: https://khanacademy.atlassian.net/browse/INFRA-9497

## Test plan:
None

Author: csilvers

Reviewers: dbraley, MiguelCastillo, nathanjd

Required Reviewers:

Approved By: dbraley, MiguelCastillo

Checks:

Pull Request URL: #90

containers/kabuild/Dockerfile

@@ -64,7 +64,7 @@ RUN apt-get install -y \
 
 # TODO: Get version from common location
 # WARNING: We REALLY WANT to avoid requring make and/or break standalone skaffold.
-ENV DESIRED_GO_VERSION=1.16
+ENV DESIRED_GO_VERSION=1.21
 RUN apt-get install -y golang-${DESIRED_GO_VERSION}
 RUN cp -sf /usr/lib/go-${DESIRED_GO_VERSION}/bin/* /usr/local/bin/
 

shared-functions.sh

@@ -134,7 +134,7 @@ install_protoc_common() {
 }
 
 DESIRED_GO_MAJOR_VERISON=1
-DESIRED_GO_MINOR_VERISON=19
+DESIRED_GO_MINOR_VERISON=21
 DESIRED_GO_VERSION="$DESIRED_GO_MAJOR_VERISON.$DESIRED_GO_MINOR_VERISON"
 
 # Evaluates to truthy if go is installed and