Keep the main performance channel on the .NET 11 SDK

[LoopedBard3]

Summary

Change only scripts/channel_map.py so main.branch selects 11.0 instead of 12.0, partially reverting dotnet#5299. Keep the net11.0 TFM, .NET 12 NuGet feeds, and explicit .NET 11 channel mappings unchanged.

Why

The early .NET 12 SDK runs the BenchmarkDotNet host on .NET 12 but still supports targeting only through net11.0. BDN's CoreRun toolchain derives its generated target framework from the host runtime, so it generates net12.0 projects that fail with NETSDK1045.

The runtime performance pipeline selects and pins the SDK during setup, then passes its exact version to Helix. Changing only the TFM-to-channel reverse lookup would not fix that path; rolling back the main SDK channel does.

Validation

• Checked that forward and reverse mappings select SDK branch 11.0 for net11.0, and that all other mappings remain unchanged.
• Ran internal dotnet-runtime-perf sanity build 3072516 with onlySanityCheck=true, using this exact fix commit (404aa6a669466c71cb7070f688adcda46593d63d).
• Windows x64 CoreRun used SDK 11.0.100-rc.2.26460.105, successfully restored/published the BDN-generated net11.0 project, and executed 35 benchmarks with real measurements and no NETSDK1045. The second Windows x64 leg also executed 35 benchmarks; Windows x86 passed.
• Overall, 23 of 25 Helix jobs passed. The pipeline finished red on two distinct failures: the ARM64 SVE sanity filter selects no benchmarks and artifact collection then fails; CoreCLR WASM R2R linking requires runtime 12.0.0-ci, while the selected installation contains 12.0.0-alpha.1.26458.117, causing MSB6006/NETSDK1144. Neither is the original CoreRun NETSDK1045 failure.