Update dependency proxy-agent to v4

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
proxy-agent	^3.0.3 -> ^4.0.0

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Release Notes

TooTallNate/node-proxy-agent

v4.0.1

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Patches

• README.md: Mention proxy-from-env support: #​56
• Add Node.js 14 to testing matrix: #​58
• Fix type definitions to account for transparent proxy discovery: #​57

Credits

Huge thanks to @​nhooyr and @​nulltoken for helping!

v4.0.0

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Major Changes

• Update agent-base to v6: #​55

Credits

Huge thanks to @​davidpatrick for helping!

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