Fix firmware release workflow: build against ESP-IDF v6.0, not v5.3

The build container was pinned to espressif/idf:release-v5.3, but the
firmware is actually developed against v6.0.2 (this machine's
$IDF_PATH). That gap caused real compile failures in CI --
esp_sleep_enable_gpio_wakeup_on_hp_periph_powerdown and
ESP_NETIF_CAPTIVEPORTAL_URI are both newer than v5.3 has, plus a
transitive stdbool.h include that changed between versions -- none of
it a real bug in the source.

.gitea/workflows/firmware-release-build.yml

@@ -56,7 +56,7 @@ jobs:
       - name: Build (devkit -- ESP32-C6-DevKitC-1)
         run: |
           mkdir -p /tmp/release-assets
-          cid=$(docker create -w /workspace/firmware espressif/idf:release-v5.3 bash -c '
+          cid=$(docker create -w /workspace/firmware espressif/idf:release-v6.0 bash -c '
             git config --global --add safe.directory /workspace &&
             . "$IDF_PATH/export.sh" &&
             ./build_for_board.sh devkit set-target esp32c6 &&
@@ -69,7 +69,7 @@ jobs:
 
       - name: Build (xiao -- Seeed XIAO ESP32-C6)
         run: |
-          cid=$(docker create -w /workspace/firmware espressif/idf:release-v5.3 bash -c '
+          cid=$(docker create -w /workspace/firmware espressif/idf:release-v6.0 bash -c '
             git config --global --add safe.directory /workspace &&
             . "$IDF_PATH/export.sh" &&
             ./build_for_board.sh xiao set-target esp32c6 &&