NOTE: this ticket can't be done until Mongo 8.X is EOL.

As part of removing log-based incremental backup, the testing and example code was removed in WT-12469. But the upgrade/downgrade compatibility testing runs test/format with older WiredTiger versions, which could generate config with backup.incremental=log. There's a little bit of code left to handle this (https://github.com/wiredtiger/wiredtiger/pull/10405/files#diff-697c4698297db509ea7fd0ead993bc92c12a9e68cb1b6497a2385ac06087ee54R2248) which can go away once the oldest version of WiredTiger that the upgrade/downgrade testing uses corresponds to MongoDB >= 9.0.