fix: bump proxy-agent to 6.4.0

[VarunWachaspati]

• Bumps proxy-agent to 6.4.0 to address the CVE-2023-42282
• Resolves https://github.com/box/box-node-sdk/issues/873

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Pull Request Test Coverage Report for Build 7898190592

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.503%

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Totals
Change from base Build 7459720049:	0.0%
Covered Lines:	2955
Relevant Lines:	3064

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💛 - Coveralls