Clean up stale Ubuntu references in comments and docs

docs/workflow/building/coreclr/cross-building.md

@@ -168,7 +168,7 @@ docker run --rm \
   -v <RUNTIME_REPO_PATH>:/runtime \
   -w /runtime \
   -e ROOTFS_DIR=/crossrootfs/x64 \
-  mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-18.04-cross-freebsd-12 \
+  mcr.microsoft.com/dotnet-buildtools/prereqs:azurelinux-3.0-net11.0-cross-freebsd-14-amd64 \
   ./build.sh --subset clr --cross --os freebsd
 ```
 

docs/workflow/building/libraries/webassembly-instructions.md

@@ -119,42 +119,13 @@ L: GC_MAJOR: (user request) time 3.00ms, stw 3.00ms los size: 0K in use: 0K
 
 ## Updating Emscripten version in Docker image
 
-First update emscripten version in the [webassembly Dockerfile](https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/master/src/ubuntu/18.04/webassembly/Dockerfile#L19).
+First update the emscripten version in the [webassembly Dockerfile](https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/main/src/ubuntu/24.04/helix/webassembly/amd64/Dockerfile).
 
-```
-ENV EMSCRIPTEN_VERSION=1.39.16
-```
-
-Submit a PR request with the updated version, wait for all checks to pass and for the request to be merged. A [master.json file](https://github.com/dotnet/versions/blob/master/build-info/docker/image-info.dotnet-dotnet-buildtools-prereqs-docker-master.json#L1126) will be updated with the a new docker image.
+Submit a PR with the updated version, wait for all checks to pass and for it to be merged. The [image-info manifest](https://github.com/dotnet/versions/blob/main/build-info/docker/image-info.dotnet-dotnet-buildtools-prereqs-docker-main.json) will be updated with the new docker image tag.
 
-```
-{
-  "platforms": [
-    {
-      "dockerfile": "src/ubuntu/18.04/webassembly/Dockerfile",
-      "simpleTags": [
-        "ubuntu-18.04-webassembly-20210707133424-12f133e"
-      ],
-      "digest": "sha256:1f2d920a70bd8d55bbb329e87c3bd732ef930d64ff288dab4af0aa700c25cfaf",
-      "osType": "Linux",
-      "osVersion": "Ubuntu 18.04",
-      "architecture": "amd64",
-      "created": "2020-05-29T22:16:52.5716294Z",
-      "commitUrl": "https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/6a6da637580ec557fd3708f86291f3ead2422697/src/ubuntu/18.04/webassembly/Dockerfile"
-    }
-  ]
-},
-```
-
-Copy the docker image tag and replace it in [platform-matrix.yml](https://github.com/dotnet/runtime/blob/main/eng/pipelines/common/platform-matrix.yml#L172)
-
-```
-container:
-    image: ubuntu-18.04-webassembly-20210707133424-12f133e
-    registry: mcr
-```
+Copy the docker image tag and update the references in the runtime repo pipeline files (e.g., `eng/pipelines/helix-platforms.yml` and `eng/pipelines/libraries/helix-queues-setup.yml`).
 
-Open a PR request with the new image.
+Open a PR with the new image.
 
 # Test libraries
 

src/libraries/Common/tests/System/Net/EnterpriseTests/setup/docker-compose.yml

@@ -1,5 +1,3 @@
-version: "3.7"
-
 services:
   kdc:
     build:

src/libraries/System.Net.Http/tests/StressTests/HttpStress/docker-compose.yml

@@ -1,4 +1,3 @@
-version: '3' # Although the version attribute is obsolete and should be ignored, it's seemingly not the case on Build.Ubuntu.2204.Amd64.Open
 services:
   client:
     build:

src/libraries/System.Net.Security/tests/StressTests/SslStress/docker-compose.yml

@@ -1,4 +1,3 @@
-version: '3' # Although the version attribute is obsolete and should be ignored, it's seemingly not the case on Build.Ubuntu.2204.Amd64.Open
 services:
   client:
     build: