[CI] Enable dependabot to bump selected dependencies

[novusnota]

Let's open automatic PRs when there is an update to mint or some other dependencies that might require attention. Preferably with a dependabot from GitHub.

See:

• https://docs.github.com/en/code-security/getting-started/dependabot-quickstart-guide#enabling-dependabot-for-your-repository
• https://github.com/tact-lang/tact/blob/main/.github/dependabot.yml