Enable Dependabot

[rootulp]

Context

https://github.com/celestiaorg/nmt/blob/33110363c1d9bc966c1a6f22fd6c35ff41f00b31/.github/workflows/go.yml#L16

is outdated because there exists a setup-go v4.1.0.

Problem

Dependabot is not configured for this repo

Proposal

Enable dependabot for this repo. Inspiration can be drawn from

• https://github.com/celestiaorg/celestia-app/blob/0d1750a40d34d2788b3f3a0a8d084f2dad76eb5c/.github/dependabot.yml
• https://github.com/celestiaorg/rsmt2d/blob/39f259f21a4254fa8fdf4b9131df777e1c7f0d3a/.github/dependabot.yml

[Chirag018]

@rootulp would like to work on this