Update module pion/webrtc/v2 to v2.1.18

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
github.com/pion/webrtc/v2	require	patch	v2.1.17 -> v2.1.18

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Release Notes

pion/webrtc

### [`v2.1.18`](https://togithub.com/pion/webrtc/compare/v2.1.17...v2.1.18) [Compare Source](https://togithub.com/pion/webrtc/compare/v2.1.17...v2.1.18)

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