Automated steps

• Submit a PR targeting main that mimics what was done in update version prefix to 18.3 #12786 but for vs18.4 instead of vs18.3.
• Update the PackageValidationBaselineVersion property in eng/Versions.props to the latest version of the Microsoft.Build package that is available on nuget.org in that PR.
• Update the .config/git-merge-flow-config.jsonc file and add or update a "vs18.3" element that has a MergeToBranch key pointing to the "main" branch. Update the vs18.0 element's MergeToBranch key to "vs18.3".

Manual steps

• Create a "vs18.3" branch with https://github.com/dotnet/msbuild/tree/fb775842a00d534aaf25466ff76e0d8fe86d2e7d as the HEAD SHA
• Create a darc default channel for msbuild's vs18.3 branch that maps to the "VS 18.3" channel: darc add-default-channel --channel "VS 18.3" --branch "vs18.3" --repo https://github.com/dotnet/msbuild
• Update the darc default channel for msbuild's main branch to "VS 18.4": darc delete-default-channel --channel "VS 18.3" --branch "main" --repo https://github.com/dotnet/msbuild && darc add-default-channel --channel "VS 18.4" --branch "main" --repo https://github.com/dotnet/msbuild
• Add an arcade ( .NET 10 Eng) --> msbuild (vs18.3) subscription
• Add a VMR (.NET 10.0.2xx SDK) -> msbuild (vs18.3) subscription
• Add an msbuild (VS 18.3) --> VMR (release/10.0.2xx) subscription
• After the mirror synced to the internal branch, trigger an MSBuild-OptProf run for vs18.3: https://devdiv.visualstudio.com/DevDiv/DevDiv%20Team/_build?definitionId=17389&_a=summary