fix: add missing gradle and devcontainers to package_managers_found

Pre-existing inconsistency where these ecosystems were detected but
not initialized in the package_managers_found dict. Works due to
Python dict behavior but inconsistent with all other ecosystems.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: zkoppert

dependabot_file.py

@@ -224,6 +224,8 @@ def build_dependabot_file(
         "terraform": False,
         "github-actions": False,
         "maven": False,
+        "gradle": False,
+        "devcontainers": False,
         "bazel": False,
         "bun": False,
         "conda": False,