GH-41147: [CI][C++] Use newer LLVM on Ubuntu 24.04 (#41150)

### What changes are included in this PR?

Use LLVM 15 on Ubuntu 24.04, as LLVM 14 packages seem not always available.

### Are these changes tested?

Yes, on CI.

### Are there any user-facing changes?

No.
* GitHub Issue: #41147

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>

ci/docker/ubuntu-24.04-cpp.dockerfile

@@ -29,7 +29,7 @@ RUN echo "debconf debconf/frontend select Noninteractive" | \
 # while debugging package list with docker build.
 ARG clang_tools
 ARG llvm
-RUN latest_system_llvm=14 && \
+RUN latest_system_llvm=18 && \
     if [ ${llvm} -gt ${latest_system_llvm} -o \
          ${clang_tools} -gt ${latest_system_llvm} ]; then \
       apt-get update -y -q && \
@@ -127,7 +127,7 @@ RUN if [ "${gcc_version}" = "" ]; then \
           g++ \
           gcc; \
     else \
-      if [ "${gcc_version}" -gt "12" ]; then \
+      if [ "${gcc_version}" -gt "14" ]; then \
           apt-get update -y -q && \
           apt-get install -y -q --no-install-recommends software-properties-common && \
           add-apt-repository ppa:ubuntu-toolchain-r/volatile; \

dev/tasks/tasks.yml

@@ -1055,7 +1055,7 @@ tasks:
     params:
       image: conda-cpp-valgrind
 
-{% for ubuntu_version in ["20.04", "22.04", "24.04"] %}
+{% for ubuntu_version in ["20.04", "22.04"] %}
   test-ubuntu-{{ ubuntu_version }}-cpp:
     ci: github
     template: docker-tests/github.linux.yml
@@ -1073,13 +1073,25 @@ tasks:
         UBUNTU: 20.04
       image: ubuntu-cpp-bundled
 
+  test-ubuntu-24.04-cpp:
+    ci: github
+    template: docker-tests/github.linux.yml
+    params:
+      env:
+        CLANG_TOOLS: 15
+        LLVM: 15
+        UBUNTU: 24.04
+      image: ubuntu-cpp
+
   test-ubuntu-24.04-cpp-gcc-14:
     ci: github
     template: docker-tests/github.linux.yml
     params:
       env:
-        UBUNTU: "24.04"
+        CLANG_TOOLS: 15
         GCC_VERSION: 14
+        LLVM: 15
+        UBUNTU: 24.04
       # rapidjson 1.1.0 has an error caught by gcc 14.
       # https://github.com/Tencent/rapidjson/issues/718
       flags: -e CC=gcc-14 -e CXX=g++-14 -e RapidJSON_SOURCE=BUNDLED