Opt-in Dependabot version update configuration

The `.github/dependabot.yml` Dependabot configuration file [2] for
automation version updates [1] that was introduced in GH-52 [3] often
causes a lot of PR noise and does not really help since updates also
often require more action than just a bump of the version number itself
like migration steps or adjustments to changes (e.g. APIs or deprecated
implementations). Since Dependabot is not able to fulfill this and only
does a stupid increase of the version number it often creates more work
than it helps. The result are often hundreds of notifications and more
digital noise for developers and maintainers without any real benefit
since version & security updates are done on a regular schedule by
maintainers who know what they are doing and how modern software should
be maintained.
Therefore the `.github/dependabot.yml` file has been renamed to
`.github/dependabot.tmpl.yml` to disable Dependabot for this repository
while still allowing repositories that are based on this template
repository to opt-in.

[1]: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/about-dependabot-version-updates
[2]: https://github.com/svengreb/tmpl/blob/32925a1f/.github/dependabot.yml
[3]: #52

GH-94