build(deps): bump github.com/pion/rtp from 1.8.20 to 1.8.21 (#4727)

Bumps [github.com/pion/rtp](https://github.com/pion/rtp) from 1.8.20 to 1.8.21.
- [Release notes](https://github.com/pion/rtp/releases)
- [Changelog](https://github.com/pion/rtp/blob/master/.goreleaser.yml)
- [Commits](https://github.com/pion/rtp/compare/v1.8.20...v1.8.21)

---
updated-dependencies:
- dependency-name: github.com/pion/rtp
  dependency-version: 1.8.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

go.mod

@@ -28,7 +28,7 @@ require (
 	github.com/pion/interceptor v0.1.40
 	github.com/pion/logging v0.2.4
 	github.com/pion/rtcp v1.2.15
-	github.com/pion/rtp v1.8.20
+	github.com/pion/rtp v1.8.21
 	github.com/pion/sdp/v3 v3.0.14
 	github.com/pion/webrtc/v4 v4.0.7
 	github.com/stretchr/testify v1.10.0

go.sum

@@ -167,8 +167,8 @@ github.com/pion/randutil v0.1.0 h1:CFG1UdESneORglEsnimhUjf33Rwjubwj6xfiOXBa3mA=
 github.com/pion/randutil v0.1.0/go.mod h1:XcJrSMMbbMRhASFVOlj/5hQial/Y8oH/HVo7TBZq+j8=
 github.com/pion/rtcp v1.2.15 h1:LZQi2JbdipLOj4eBjK4wlVoQWfrZbh3Q6eHtWtJBZBo=
 github.com/pion/rtcp v1.2.15/go.mod h1:jlGuAjHMEXwMUHK78RgX0UmEJFV4zUKOFHR7OP+D3D0=
-github.com/pion/rtp v1.8.20 h1:8zcyqohadZE8FCBeGdyEvHiclPIezcwRQH9zfapFyYI=
-github.com/pion/rtp v1.8.20/go.mod h1:bAu2UFKScgzyFqvUKmbvzSdPr+NGbZtv6UB2hesqXBk=
+github.com/pion/rtp v1.8.21 h1:3yrOwmZFyUpcIosNcWRpQaU+UXIJ6yxLuJ8Bx0mw37Y=
+github.com/pion/rtp v1.8.21/go.mod h1:bAu2UFKScgzyFqvUKmbvzSdPr+NGbZtv6UB2hesqXBk=
 github.com/pion/sctp v1.8.36 h1:owNudmnz1xmhfYje5L/FCav3V9wpPRePHle3Zi+P+M0=
 github.com/pion/sctp v1.8.36/go.mod h1:cNiLdchXra8fHQwmIoqw0MbLLMs+f7uQ+dGMG2gWebE=
 github.com/pion/sdp/v3 v3.0.14 h1:1h7gBr9FhOWH5GjWWY5lcw/U85MtdcibTyt/o6RxRUI=