Fix duplicate log messages

[kamilkrzyskow]

MkDocs 1.5 started using f-strings when creating build time log messages, therefore each string is unique (as the time typically changes slightly).
The old solution stopped working, so now instead I've created a different DuplicateFilter class, which looks at the prefix of the log and not the exact match.
Additionally this PR changes makes more logging changes:

• adds messages about default language being built
• scopes the lunr.js log warning only to the built language

[ultrabug]

Thanks a lot, almost there

[ultrabug]

Thank you!