ci: Use macos-11 in all CI (#5493)

### Description
Addresses two issues:
1. The nightly builds are starting to occasionally fail because the
`macos-latest` runners are struggling to complete integration test
shards in under 2 hours. It seems likely the newer ARM-based runners are
underpowered.
2. ~`macos-11` runners will be removed by 6/28, and will start
temporarily failing on 6/17. Better to move off them now.~ `macos-12`
hits a compiler error on DAST.cs for some reason (`error CS8078: An
expression is too long or complex to compile`) so we'll stick to
`macos-11` for this PR

### How has this been tested?
`run-deep-tests` label

<small>By submitting this pull request, I confirm that my contribution
is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

.github/workflows/integration-tests-reusable.yml

@@ -27,15 +27,15 @@ jobs:
       - name: Populate OS list (all platforms)
         id: populate-os-list-all
         if: inputs.all_platforms
-        run: echo "os-list=[\"ubuntu-latest\", \"ubuntu-20.04\", \"macos-latest\", \"windows-2019\"]" >> $GITHUB_OUTPUT
+        run: echo "os-list=[\"ubuntu-latest\", \"ubuntu-20.04\", \"macos-11\", \"windows-2019\"]" >> $GITHUB_OUTPUT
       - name: Populate OS list (one platform)
         id: populate-os-list-one
         if: "!inputs.all_platforms"
         run: echo "os-list=[\"ubuntu-20.04\"]" >> $GITHUB_OUTPUT
       - name: Populate OS mapping for package.py
         id: populate-os-mapping
         run: |
-          echo "os-mapping={\"ubuntu-latest\": \"ubuntu\", \"ubuntu-20.04\": \"ubuntu\", \"macos-latest\": \"macos\", \"windows-2019\": \"windows\"}" >> $GITHUB_OUTPUT
+          echo "os-mapping={\"ubuntu-latest\": \"ubuntu\", \"ubuntu-20.04\": \"ubuntu\", \"macos-11\": \"macos\", \"windows-2019\": \"windows\"}" >> $GITHUB_OUTPUT
       - name: Populate target runtime version list (all platforms)
         id: populate-target-runtime-version-all
         if: inputs.all_platforms

.github/workflows/publish-release-reusable.yml

@@ -35,7 +35,7 @@ env:
 jobs:
 
   publish-release:
-    runs-on: macos-latest   # Put back 'ubuntu-20.04' if macos-latest fails in any way
+    runs-on: macos-11   # Put back 'ubuntu-20.04' if macos-11 fails in any way
     steps:
     - name: Print version
       run: echo ${{ inputs.name }}

.github/workflows/release-brew.yml

@@ -10,7 +10,7 @@ concurrency:
 jobs:
   build:
 
-    runs-on: macos-latest
+    runs-on: macos-11
 
     steps:
     - name: Install dafny

.github/workflows/release-downloads-nuget.yml

@@ -111,7 +111,7 @@ jobs:
         # but note we need to skip Dafny since nuget install doesn't work for dotnet tools.
         library-name: [ DafnyPipeline, DafnyServer, DafnyLanguageServer, DafnyRuntime, DafnyCore, DafnyDriver ]
         # This workflow breaks on windows-2022: https://github.com/dafny-lang/dafny/issues/1906
-        os: [ ubuntu-latest, ubuntu-20.04, macos-latest, windows-2019 ]
+        os: [ ubuntu-latest, ubuntu-20.04, macos-11, windows-2019 ]
 
     steps:
     # Verify that the dependencies of the libraries we publish (e.g. DafnyLanguageServer)

.github/workflows/release-downloads.yml

@@ -18,13 +18,13 @@ jobs:
       fail-fast: false
       matrix:
         # This workflow breaks on windows-2022: https://github.com/dafny-lang/dafny/issues/1906
-        os: [ ubuntu-latest, ubuntu-20.04, macos-latest, windows-2019 ]
+        os: [ ubuntu-latest, ubuntu-20.04, macos-11, windows-2019 ]
         include:
         - os:  'ubuntu-latest'
           osn: 'ubuntu-20.04'
         - os:  'ubuntu-20.04'
           osn: 'ubuntu-20.04'
-        - os:  'macos-latest'
+        - os:  'macos-11'
           osn: 'x64-macos-11'
         - os:  'windows-2019'
           osn: 'windows-2019'