Stop using Microsoft/GitHub's `ubuntu-*` images as the linux-x64 build image for .NET pipelines

[lbussell]

Today, we use the following default images for linux-amd64 builds:

• Public: ubuntu-latest
• Internal: 1es-ubuntu-2204

Downsides of using these images:

• We (.NET) do not control the contents of these images. And they contain a lot of stuff.
• Using them causes slower builds in two ways
  • Cleaning off the pre-loaded docker images takes extra time
  • Public PR validation using the default pool ("Azure Pipelines") and the Microsoft-hosted images is limited to 200 concurrent jobs across the entire dotnet org. Oftentimes we have to wait for agents during PR validation. We have much more capacity in other pools like NetCore-Public. Available pools can be found at https://dev.azure.com/dnceng-public/public/_settings/agentqueues

To me it looks like we should use the following images, until Azure Linux 3 images are available from dnceng:

• Public: build.ubuntu.2204.amd64.open
• Internal: build.ubuntu.2204.amd64

See the following code:

docker-tools/eng/common/templates/variables/dotnet/common.yml

Lines 21 to 22 in 9955458

	- name: linuxAmd64InternalPoolImage
	value: 1es-ubuntu-2204

docker-tools/eng/common/templates/variables/common.yml

Lines 68 to 69 in 9955458

	- name: defaultLinuxAmd64PoolImage
	value: ubuntu-latest

docker-tools/eng/common/templates/stages/dotnet/publish.yml

Lines 48 to 56 in 9955458

	pool:
	${{ if ne(parameters.pool, '') }}:
	${{ parameters.pool }}
	${{ elseif eq(variables['System.TeamProject'], parameters.publicProjectName) }}:
	vmImage: $(defaultLinuxAmd64PoolImage)
	${{ elseif eq(variables['System.TeamProject'], parameters.internalProjectName) }}:
	name: $(linuxAmd64InternalPoolName)
	image: $(linuxAmd64InternalPoolImage)
	os: linux

docker-tools/eng/common/templates/stages/dotnet/build-and-test.yml

Lines 57 to 66 in 9955458

	# Linux AMD64
	linuxAmd64Pool:
	${{ if ne(parameters.linuxAmd64Pool, '') }}:
	${{ parameters.linuxAmd64Pool }}
	${{ elseif eq(variables['System.TeamProject'], parameters.publicProjectName) }}:
	vmImage: $(defaultLinuxAmd64PoolImage)
	${{ elseif eq(variables['System.TeamProject'], parameters.internalProjectName) }}:
	name: $(linuxAmd64InternalPoolName)
	image: $(linuxAmd64InternalPoolImage)
	os: linux

[lbussell]

Moved out of sprint because this is blocked by dotnet/dnceng#5027.

[lbussell]

[Triage] There is now also a 1es-azurelinux-3 image that we should prefer to use over another Ubuntu image if possible. We are currently blocked from using it due to dotnet/dnceng#6029.