Bump soupsieve from 2.5 to 2.8.4 in /.test-infra/mock-apis (#39315)

Bumps [soupsieve](https://github.com/facelessuser/soupsieve) from 2.5 to 2.8.4.
- [Release notes](https://github.com/facelessuser/soupsieve/releases)
- [Commits](facelessuser/soupsieve@2.5...2.8.4)

---
updated-dependencies:
- dependency-name: soupsieve
  dependency-version: 2.8.4
  dependency-type: indirect
...

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