Update dependency node-fetch to v2.6.8

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
node-fetch	2.6.7 -> 2.6.8

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Release Notes

node-fetch/node-fetch

### [`v2.6.8`](https://togithub.com/node-fetch/node-fetch/releases/tag/v2.6.8) [Compare Source](https://togithub.com/node-fetch/node-fetch/compare/v2.6.7...v2.6.8) ##### Bug Fixes - **headers:** don't forward secure headers on protocol change ([#​1605](https://togithub.com/bitinn/node-fetch/issues/1605)) ([fddad0e](https://togithub.com/bitinn/node-fetch/commit/fddad0e7ea3fd6da01cc006fdf0ed304ccdd7990)), closes [#​1599](https://togithub.com/bitinn/node-fetch/issues/1599) - premature close with chunked transfer encoding and for async iterators in Node 12 ([#​1172](https://togithub.com/bitinn/node-fetch/issues/1172)) ([50536d1](https://togithub.com/bitinn/node-fetch/commit/50536d1e02ad42bdf262381034805378b98bfa53)), closes [#​1064](https://togithub.com/bitinn/node-fetch/issues/1064) [/github.com/node-fetch/node-fetch/pull/1064#issuecomment-849167400](https://togithub.com//github.com/node-fetch/node-fetch/pull/1064/issues/issuecomment-849167400) - prevent hoisting of the undefined `global` variable in `browser.js` ([#​1534](https://togithub.com/bitinn/node-fetch/issues/1534)) ([8bb6e31](https://togithub.com/bitinn/node-fetch/commit/8bb6e317c866c4134e7d67e90a5596a8c67e3965))

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