🔧 [dependabot] Reduce merge conflicts using the lockfile-only strategy

Dependabot updates version constraints in pyproject.toml even when the version
constraint already covers the new version, leading to frequent merge conflicts
because the global content hash in poetry.lock changes.

As of cjolowicz/cookiecutter-hypermodern-python-instance#744, this project
template no longer uses upper version bounds for its dependencies. As a result,
we are now able to use the "lockfile-only" versioning strategy to upgrade
dependencies, including major version bumps. This strategy prevents Dependabot
from modifying pyproject.toml, putting an end to the frequent merge conflicts.

Retrocookie-Original-Commit: cjolowicz/cookiecutter-hypermodern-python-instance@3082062

{{cookiecutter.project_name}}/.github/dependabot.yml

@@ -16,3 +16,4 @@ updates:
     directory: "/"
     schedule:
       interval: daily
+    versioning-strategy: lockfile-only