Add dependabot configuration file

[jrfnl]

This commit adds an initial Dependabot configuration to:

• Submit pull requests for security updates and version updates for GH Action runner dependencies.

This PR doesn't include a config for the Composer dependencies as those will always need managed updates for this repo.

The configuration has been set up to:

• Run weekly (for now).
• Submit a maximum of 5 pull requests at a time. If additional pull requests are needed, these will subsequently be submitted the next time Dependabot runs after one or more of the open pull requests have been merged.
• The commit messages for PRs submitted by Dependabot will be prefixed according the unofficial conventions used in this repo up to now.
• The PRs will automatically be labelled with an appropriate label as already in use in this repo.

Refs:

• https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
• https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#versioning-strategy