Dependabot cannot be upgraded to GitHub-native version due to unsupported `dep` package manager

Dependabot Preview will be shut down on August 3rd, 2021.

It appears that this repository has configured updates for `dep`.

The GitHub-native version of Dependabot does not support `dep`, which was [deprecated][deprecated] as of 2020.
Guidance from the `dep` maintainers is to upgrade to [go mod][go mod], which Dependabot [supports][supported ecosystems].

In order to keep getting updates for your Go dependencies, you'll need to migrate to `go mod` and add a [Dependabot configuration][Dependabot config file] for it to this repository.

[deprecated]: https://golang.github.io/dep/docs/introduction.html#:~:text=As%20of%202020%2C%20Dep%20is,official%20support%20since%20Go%201.11.&text=This%20is%20documentation%20for%20dep,tool%20for%20the%20Go%20language.
[go mod]: https://golang.org/ref/mod
[supported ecosystems]: https://docs.github.com/en/code-security/supply-chain-security/configuration-options-for-dependency-updates#package-ecosystem
[Dependabot config file]: https://docs.github.com/en/code-security/supply-chain-security/configuration-options-for-dependency-updates#about-the-dependabotyml-file

Start by locating the repository's existing dep configuration and Go dependency declarations. Migrate the dependencies to Go modules and add a Dependabot configuration for the supported Go module ecosystem; done means dependency updates continue after the legacy dep setup is removed.

Written by the indexing model from the issue text.