fix: Update `proxy-agent` usage, drop support Node 12

[congminh1254]

Closes: box/boxcli#505

While bumping the proxy-agent from version 5 to 6, the import syntax changed from:

var ProxyAgent = require('proxy-agent');

to

import { ProxyAgent } from 'proxy-agent';

And proxy-agent is not supporting Node 12 anymore.

[coveralls]

Pull Request Test Coverage Report for Build 6747352852

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.672%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/util/config.ts	0	1	0.0%
<!--	Total:	0	1	0.0%	-->

Totals
Change from base Build 6744241683:	0.0%
Covered Lines:	2956
Relevant Lines:	3058

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💛 - Coveralls