Rename from-v3.x-to-v4.0-1.md

In https://github.com/web-auth/doc/blob/v5.0/migration/from-v3.x-to-v4.0-1.md says `From 5.x to 6.0` while having a description of `Step-by-step guide for migrating from 4.x to 5.0`

In https://github.com/web-auth/doc/blob/v4.9/migration/from-v3.x-to-v4.0-1.md it says `From 4.x to 5.0`

etc

Can we name the file to match the version changes?