Setup dependabot for example repositories

[Alexey Palazhchenko]

• https://github.com/FerretDB/dotnet-example
• https://github.com/FerretDB/java-example
• https://github.com/FerretDB/python-example

Those repos contain simple tests for language drivers, but there are no dependabot configurations that ensure that we test the latest versions. We should add them.

We also should add requirements.txt to the Python example as this repo does not use any dependency manager at all.

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