Do less CI customization, now that windows-11-arm is updated

The `windows-11-arm` image has been updated with more of the usual
software present on GitHub-hosted GitHub Actions runner images for
Windows, at current versions.

- Git for Windows is updated to a version not suffering from GitoxideLabs#1849.
  So this removes the steps that had upgraded Git for Windows and
  checked the version before and after the upgrade.

- `rustup` has been added. So this removes the helper action that
  installed it, and removes the `uses` step for it from the two
  jobs that had called it.

(If either of these is needed in the future, on this or any other
job, then they can be brought back from the commit history.)

Author: EliahKagan

.github/actions/setup-windows-arm-rustup/action.yml

@@ -221,9 +221,6 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
-      - name: Install rustup
-        if: matrix.os == 'windows-11-arm'
-        uses: ./.github/actions/setup-windows-arm-rustup
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - name: cargo check default features
@@ -253,44 +250,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - name: Report version/build of Git in this runner image
-        run: git version --build-options
-      - name: Upgrade Git for Windows to latest stable release
-        # TODO(ci): Remove this and related steps once `windows-11-arm` ships a new enough Git.
-        # The Windows 11 ARM runner still ships Git 2.48.*, so it remains affected by #1849.
-        if: matrix.os == 'windows-11-arm'
-        env:
-          GH_TOKEN: ${{ github.token }}
-        run: |
-          $workingDir = '~/git-dl'
-          $repo = 'git-for-windows/git'
-          $pattern = 'Git-*-arm64.exe'
-          $log = 'setup-log.txt'
-          # Inno Setup args reference: https://jrsoftware.org/ishelp/index.php?topic=setupcmdline
-          $arguments = @(
-            '/VERYSILENT',
-            '/SUPPRESSMSGBOXES',
-            '/ALLUSERS',
-            "/LOG=$log",
-            '/NORESTART',
-            '/CLOSEAPPLICATIONS',
-            '/FORCECLOSEAPPLICATIONS'
-          )
-
-          mkdir $workingDir | Out-Null
-          cd $workingDir
-          gh release download --repo $repo --pattern $pattern
-          $installer = Get-Item $pattern
-          Start-Process -FilePath $installer -ArgumentList $arguments -NoNewWindow -Wait
-
-          Get-Content -Path $log -Tail 50
-      - name: Report version/build of Git after upgrade
-        if: matrix.os == 'windows-11-arm'
-        run: git version --build-options
       - uses: actions/checkout@v4
-      - name: Install rustup
-        if: matrix.os == 'windows-11-arm'
-        uses: ./.github/actions/setup-windows-arm-rustup
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - uses: taiki-e/install-action@v2