CI: Bump to Ubuntu Linux 24.04

Ubuntu Linux 24.04 (noble) is the latest LTS release, and this commit
updates the CI pipeline hosts and recommended development environments
accordingly. The version of clang-format is updated to v18, with all C
source files re-indented.

In addition, the contribution guidance was synchronized with the
upstream changes.

Author: jserv

.ci/check-format.sh

@@ -6,7 +6,7 @@ set -x
 
 for file in ${SOURCES};
 do
-    clang-format-12 ${file} > expected-format
+    clang-format-18 ${file} > expected-format
     diff -u -p --label="${file}" --label="expected coding style" ${file} expected-format
 done
-exit $(clang-format-12 --output-replacements-xml ${SOURCES} | egrep -c "</replacement>")
+exit $(clang-format-18 --output-replacements-xml ${SOURCES} | egrep -c "</replacement>")

.github/workflows/main.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   semu-linux:
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-24.04
     steps:
     - uses: actions/checkout@v4
     - name: install-dependencies
@@ -37,11 +37,11 @@ jobs:
       shell: bash
 
   coding_style:
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-24.04
     steps:
     - uses: actions/checkout@v4
     - name: coding convention
       run: |
-            sudo apt-get install -q -y clang-format-12
+            sudo apt-get install -q -y clang-format-18
             .ci/check-format.sh
       shell: bash