Move to clang 21 as mininum version and use it in CI (#7779)

Clang 21 is now the latest version available in Ubuntu LTS, so we can
move to it.

docs/project/contribution_tools.md

@@ -49,8 +49,8 @@ These commands should help set up a development environment on your machine.
 # Update apt.
 sudo apt update
 
-# Check that the `clang` version is at least 19, our minimum version. That needs
-# the number of the `:` in the output to be over 19. For example, `1:19.0-1`.
+# Check that the `clang` version is at least 21, our minimum version. That needs
+# the number of the `:` in the output to be over 21. For example, `1:21.0-1`.
 apt-cache show clang | grep 'Version:'
 
 # Install tools. Use the same version as the Clang version you found above. Do
@@ -88,28 +88,28 @@ it to your `$PATH`, and aliasing `bazel` to it.
 
 #### Old `clang` versions
 
-If the version of `clang` is earlier than 19, you may still have version 19
+If the version of `clang` is earlier than 21, you may still have version 21
 available. You can use the following install instead:
 
 ```shell
 # Install explicitly versioned Clang tools.
 sudo apt install \
-  clang-19 \
-  libc++-19-dev \
-  libc++abi-19-dev \
-  lld-19 \
-  lldb-19
+  clang-21 \
+  libc++-21-dev \
+  libc++abi-21-dev \
+  lld-21 \
+  lldb-21
 
 # In your Carbon checkout, tell Bazel where to find `clang`. You can also
 # export this path as the `CC` environment variable, or add it directly to
 # your `PATH`.
-echo "build --repo_env=CC=$(readlink -f $(which clang-19))" >> user.bazelrc
+echo "build --repo_env=CC=$(readlink -f $(which clang-21))" >> user.bazelrc
 ```
 
 And if it's not available directly from the distribution, you can install Clang
 tools on Debian/Ubuntu from <https://apt.llvm.org>.
 
-> NOTE: Most LLVM 19+ installs should build Carbon. If you're having issues, see
+> NOTE: Most LLVM 21+ installs should build Carbon. If you're having issues, see
 > [troubleshooting build issues](#troubleshooting-build-issues).
 
 ### macOS
@@ -203,7 +203,7 @@ These tools are essential for work on Carbon.
         -   [Bazelisk](https://docs.bazel.build/versions/master/install-bazelisk.html):
             Downloads and runs the [configured Bazel version](/.bazelversion).
     -   [Clang](https://clang.llvm.org/) and [LLVM](https://llvm.org/)
-        -   NOTE: Most LLVM 19+ installs should build Carbon. If you're having
+        -   NOTE: Most LLVM 21+ installs should build Carbon. If you're having
             issues, see
             [troubleshooting build issues](#troubleshooting-build-issues).
     -   [gh CLI](https://github.com/cli/cli): Helps with GitHub.
@@ -419,8 +419,8 @@ includes things such as changing LLVM versions, or installing libc++. Running
 
 Many build issues result from the particular options `clang` and `llvm` have
 been built with, particularly when it comes to system-installed versions. If you
-run `clang --version`, you should see at least version 19. If you see an older
-version, please update, or use the special `clang-19` instructions above.
+run `clang --version`, you should see at least version 21. If you see an older
+version, please update, or use the special `clang-21` instructions above.
 
 System installs of macOS typically won't work, for example being an old LLVM
 version or missing llvm-ar; [setup commands](#setup-commands) includes LLVM from
@@ -442,7 +442,7 @@ providing the output of the following diagnostic commands:
 ```shell
 echo $CC
 which clang
-which clang-19
+which clang-21
 clang --version
 grep llvm_bindir $(bazel info workspace)/bazel-execroot/external/+clang_toolchain_extension+bazel_cc_toolchain/clang_detected_variables.bzl