Update webext-sdk dependency to 0.6.1

Background / User story

We need to upgrade to EWE 0.6.1 to fix #1247 (closed).

Milestone: https://gitlab.com/eyeo/adblockplus/abc/webext-sdk/-/milestones/20

Dependency changes

• Name: webext-sdk
• New version: 0.6.1
• Breaking changes: None.
• Other changes:
  • https://gitlab.com/eyeo/adblockplus/abc/webext-sdk/-/issues/369 - Fixed: Filter settings not restored for Firefox Private Browsing mode users.

Integration changes

• Legal: N/A
• Development: N/A

Hints for testers

Using Firefox with Private Browsing mode:

• Installing Adblock Plus 3.13 or 3.14 and then upgrading to this version should:
  • Open the first-run page.
  • Initialize filter settings for the first time.
• Installing Adblock Plus 3.12, optionally upgrading to 3.13 and/or 3.14, and then upgrading to this version should:
  • Not open the first-run page.
  • Restore filter settings from 3.12 and ignore any changes that were made in 3.13 and 3.14.