Upgrade build tools from debian buster to bullseye

[markmandel]

Looks like we're getting some blocking on CI with Debian's latest release "bullseye" / 11:

You can see it here: https://console.cloud.google.com/cloud-build/builds/3a2543f4-524b-4b1f-9720-19daae250155?project=258182270954

make[4]: Entering directory '/workspace/build'
docker build --tag=agones-build-sdk-csharp:b954dd54f8 /workspace/build/build-sdk-images/csharp --build-arg BASE_IMAGE=agones-build-sdk-base:48cd8562c3
Sending build context to Docker daemon  11.26kB
Step 1/8 : ARG BASE_IMAGE=agones-build-sdk-base:latest
Step 2/8 : FROM $BASE_IMAGE
 ---> 432b721aac95
Step 3/8 : RUN  apt-get update     && apt-get install -y wget
 ---> Running in 42fd8fbae443
Get:1 http://security.debian.org/debian-security buster/updates InRelease [65.4 kB]
Get:2 http://deb.debian.org/debian buster InRelease [122 kB]
Get:3 http://deb.debian.org/debian buster-updates InRelease [51.9 kB]
Reading package lists...
E: Repository 'http://security.debian.org/debian-security buster/updates InRelease' changed its 'Suite' value from 'stable' to 'oldstable'
E: Repository 'http://deb.debian.org/debian buster InRelease' changed its 'Suite' value from 'stable' to 'oldstable'
E: Repository 'http://deb.debian.org/debian buster-updates InRelease' changed its 'Suite' value from 'stable-updates' to 'oldstable-updates'
includes/sdk.mk:106: recipe for target 'build-build-sdk-image' failed
make[4]: Leaving directory '/workspace/build'
make[3]: *** [ensure-image] Error 2
includes/build-image.mk:54: recipe for target 'ensure-image' failed
includes/sdk.mk:123: recipe for target 'ensure-build-sdk-image' failed

Can see a workaround here: https://www.reddit.com/r/debian/comments/ca3se6/for_people_who_gets_this_error_inrelease_changed/

Will look to see if we can do a quick workaround to unstick, but seems like a good opportunity to upgrade the base image everywhere.