Change to watchdog_severity_levels breaks log function severity filter

[yorkshire-pudding]

Recently, the PR for issue backdrop/backdrop-issues/issues/4750 was merged and this changed the default output of watchdog_severity_levels() from lowercase names to capital first letter labels.

This breaks the severity filter and returns no records if a severity filter is used.
It also breaks one of the tests used.

As we won't know unless the backdrop version is 1.26.2 or over, we should try to make the function work with either.