Replace `dotenv` (unmaintained since 2020) with `dotenvy` (RUSTSEC-2021-0141)

Description

Replace the dotenv crate with dotenvy. dotenvy is a maintained fork recommended by RUSTSEC.

Motivation and Context

dotenv is unmaintained and has not received an update since 2020. dotenvy is the recommended, drop in fork.

Sources:

https://github.com/dotenv-rs/dotenv
https://rustsec.org/advisories/RUSTSEC-2021-0141.html

Dependencies

dotenvy

Type of change

[ ] Bug fix (non-breaking change which fixes an issue)

How has this been tested?

I ran the test suite with --features env-file before and after the change. The fork doesn't change the original crate's API beyond replacing dotenv with dotenvy.

Is this change properly documented?

No, but I'll edit the CHANGELOG.md and squash the commits now.

ramsayleung / rspotify

Replace `dotenv` (unmaintained since 2020) with `dotenvy` (RUSTSEC-2021-0141) #447