Update webext-sdk dependency to 0.8.1

Background / User story

There are some bug fixes in EWE 0.8.1 that we'd like to bring in.

Dependency changes

• Name: webext-sdk
• New version: 0.8.1
• Breaking changes:
  • https://gitlab.com/eyeo/adblockplus/abc/webext-sdk/-/issues/437 - Recommendation.url now corresponds to the respective URL for the extension's manifest version.
  • https://gitlab.com/eyeo/adblockplus/abc/webext-sdk/-/issues/437 - Recommendation.mv2URL no longer exists under Manifest v2.
• Other changes:
  • https://gitlab.com/eyeo/adblockplus/abc/webext-sdk/-/issues/456 - Fixed: Ignored notifications setting was not persisted.
  • https://gitlab.com/eyeo/adblockplus/abc/webext-sdk/-/issues/458 - Fixed: Subscription.homepage was missing from non-downloadable filter lists.

Integration changes

• Legal: TBD
• Development: Adjust code to changes to Recommendation.url and Recommendation.mv2URL (incl. removing the workaround we introduced in #1329 (closed)).

Hints for testers

The following issues should remain fixed:

• #1314 (closed)
• #1329 (closed)