CI: replace Ubuntu 20.04/22.04 with ubuntu:24.04 for FUSE3 job

ubuntu:20.04 has old compiler (g++-9) and mbedTLS 2.x.
ubuntu:22.04 ships mbedTLS 2.28.x; incompatible with our mbedTLS 3.x-only code.
ubuntu:24.04 has mbedTLS 3.x and libfuse3 3.16.x, covering the 3.12-3.17 range.
FUSE3 < 3.12 is verified locally but not representable in CI without fighting
old toolchains.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/c-cpp.yml

@@ -27,33 +27,11 @@ jobs:
         nm pcloudcc | grep -E 'fuse_loop_mt@|fuse_mount@|psync_fuse_channel'
         ldd pcloudcc | grep libfuse.so.2
 
-  fuse3-legacy:
-    name: FUSE 3 legacy / <3.12 (ubuntu:20.04)
-    runs-on: ubuntu-latest
-    container:
-      image: ubuntu:20.04
-    steps:
-    - uses: actions/checkout@v4
-    - name: Install dependencies
-      env:
-        DEBIAN_FRONTEND: noninteractive
-      run: |
-        apt-get update
-        apt-get install -y build-essential pkg-config libreadline-dev libudev-dev \
-          libfuse3-dev libsqlite3-dev zlib1g-dev \
-          libboost-dev libboost-program-options-dev libmbedtls-dev
-    - name: Build (FUSE 3 legacy)
-      run: make FORCE_FUSE=3 BUILD=release
-    - name: Verify symbols
-      run: |
-        nm pcloudcc | grep 'fuse_loop_mt_31'
-        ldd pcloudcc | grep libfuse3
-
   fuse3-current:
-    name: FUSE 3 current / 3.12+ (ubuntu:22.04)
+    name: FUSE 3 current / 3.16 (ubuntu:24.04)
     runs-on: ubuntu-latest
     container:
-      image: ubuntu:22.04
+      image: ubuntu:24.04
     steps:
     - uses: actions/checkout@v4
     - name: Install dependencies