97b642a introduced a regression that broke `AbstractDateAssert`
thread-safety by replacing synchronization on `DEFAULT_DATE_FORMATS`
with synchronization on the method that parses with the default
formats. Such a synchronization cannot work properly with multiple
instances of `AbstractDateAssert` because `DEFAULT_DATE_FORMATS` is
static.

This change fixes the regression by replacing the synchronization on
the method level with a `ThreadLocal` that guards access to the current
`DEFAULT_DATE_FORMATS` content.

…3881)

See #3822.

…j.core.annotation.Beta`

…rg.assertj.core.annotation.CanIgnoreReturnValue`

…ssertj.core.annotation.CheckReturnValue`