Bump FXIOS [Maintenance] Upgrade Kingfisher dependency

[lmarceau]

📜 Tickets

Jira ticket
Github ticket

💡 Description

Upgrade Kingfisher dependency from 8.2.0 to 8.8.1

📝 Checklist

• I filled in the ticket numbers and a description of my work
• I updated the PR name to follow our PR naming guidelines
• I ensured unit tests pass and wrote tests for new code
• If working on UI, I checked and implemented accessibility (Dynamic Text and VoiceOver)
• If adding telemetry, I read the data stewardship requirements and will request a data review
• If adding or modifying strings, I read the guidelines and will request a string review from l10n
• If needed, I updated documentation and added comments to complex code

[lmarceau]

testGetURLFromCacheWithEmptyCache fails, which is very suspicious since we're upgrading Kingfisher 👀 Need to have a look!