Upgrades `htmlparser2` to new major version `^8.0.0`. (#1)

[kedarchandrayan]

Summary

Closes #565

What are the specific steps to test this change?

All existing behavior should remain same.

What kind of change does this PR introduce?

• [ ] Bug fix
• [ ] New feature
• [x] Refactor
• [ ] Documentation
• [ ] Build-related changes
• [x] Other - Upgrading htmlparser2 version

Make sure the PR fulfills these requirements:

• [x] It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• [x] The changelog is updated
• [x] Related documentation has been updated
• [x] Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[kedarchandrayan]

Closing this PR in favour of the clean new PR https://github.com/apostrophecms/sanitize-html/pull/573