Downsize Miri and migration check runners (#64392)

## Summary

Move scheduler Miri tests from 16 vCPUs / 32 GiB to 4 vCPUs / 8 GiB, and
migration checks from 8 vCPUs / 16 GiB to 4 vCPUs / 8 GiB. Reuse the
existing `LINUX_MEDIUM` profile and keep test commands and coverage
unchanged.

September 16 whole-runner CPU samples showed Miri averaging about 1.3
cores with a 2.4-core sampled peak and 2.6 GiB peak RAM; migration
checks averaged 1.2 cores with a 1.8-core sampled peak and 1.3 GiB peak
RAM. These are sampled observations, not a cold-build guarantee.

## Validation

- `cargo xtask workflows`
- `cargo fmt -p xtask -- --check`
- `git diff --check`
- CI on the smaller runners is pending.

## Human review

The root repository rules require the README review notice. The human
author must remove it after review; this PR remains a draft until that
step and CI verification are complete.

Release Notes:

- N/A

.github/workflows/run_tests.yml

@@ -485,7 +485,7 @@ jobs:
     needs:
     - orchestrate
     if: needs.orchestrate.outputs.run_tests == 'true' && github.event_name != 'merge_group'
-    runs-on: namespace-profile-16x32-ubuntu-2204
+    runs-on: namespace-profile-4x8-ubuntu-2204
     steps:
     - name: steps::harden_runner
       uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411
@@ -952,7 +952,7 @@ jobs:
     needs:
     - orchestrate
     if: needs.orchestrate.outputs.run_tests == 'true'
-    runs-on: namespace-profile-8x16-ubuntu-2204
+    runs-on: namespace-profile-4x8-ubuntu-2204
     env:
       GIT_AUTHOR_NAME: Protobuf Action
       GIT_AUTHOR_EMAIL: ci@zed.dev

tooling/xtask/src/tasks/workflows/run_tests.rs

@@ -784,7 +784,7 @@ pub(crate) fn check_postgres_and_protobuf_migrations() -> NamedJob {
 
     named::job(
         release_job(&[])
-            .runs_on(runners::LINUX_LARGE)
+            .runs_on(runners::LINUX_MEDIUM)
             .add_env(("GIT_AUTHOR_NAME", "Protobuf Action"))
             .add_env(("GIT_AUTHOR_EMAIL", "ci@zed.dev"))
             .add_env(("GIT_COMMITTER_NAME", "Protobuf Action"))
@@ -818,7 +818,7 @@ fn miri_scheduler() -> NamedJob {
 
     named::job(
         release_job(&[])
-            .runs_on(runners::LINUX_DEFAULT)
+            .runs_on(runners::LINUX_MEDIUM)
             .add_step(steps::harden_runner())
             .add_step(steps::checkout_repo())
             .add_step(steps::setup_cargo_config(Platform::Linux))