chore: Upgrade proxy agent (#163)

* Fix package-lock.json

There was a merge conflict marker committed to the `package-lock.json`
in  cc0de4b, which coincidentally
upgraded the project to use `lockfileVersion` of `2`.

* Upgrade proxy-agent to latest

In the transitive dependency [`pac-resolver` there is a known vulnerability][0].
By upgrading `proxy-agent` to latest, we're also upgrading `pac-proxy-agent` and
`pac-resolver`.

From reading the changelogs of `proxy-agent`, `pac-proxy-agent` and
`pac-resolver`, it is clear that TooTallNate has been careful not to introduce
any breaking changes, other than dropping support for old node versions.

* [`proxy-agent@5.0.0`][1]
* [`pac-proxy-agent@5.0.0`][2]
* [`pac-resolver@5.0.0`][3]

By merging this, the minimum supported node version dictated by these
dependencies will be 8.

[0]: GHSA-9j49-mfvp-vmhm
[1]: https://github.com/TooTallNate/node-proxy-agent/releases/tag/5.0.0
[2]: https://github.com/TooTallNate/node-pac-proxy-agent/releases/tag/5.0.0
[3]: https://github.com/TooTallNate/node-pac-resolver/releases/tag/5.0.0

Co-authored-by: Alberto Zaccagni <montecristoh@gmail.com>