Fix: skip PolymorphicRelationsType during export

[rvdkooy]

Hi there,

This one a bit more complicated
During a whole database export I run into the following issue when also exporting plugin content-types:

• Strapi released a future plugin not long ago called: Content Releases (https://docs.strapi.io/user-docs/releases/introduction) that is enabled by default nowadays
• this plugin uses a relation of type: PolymorphicRelationsType (see: https://github.com/strapi/strapi/blob/develop/packages/core/content-releases/server/src/content-types/release-action/schema.ts#L29)
• during the export strapi-plugin-import-export-entries fails on this because it can only handle BasicRelationsTypes

in this PR, I added logic to skip PolymorphicRelationsType and only handle BasicRelationsType relations.

questions:

• I'm not sure if this is the correct thing to do, I know too little about how the PolymorphicRelationsType should be handled.
• I looked into disabling the Content Releases plugin, but I couldn't find anything on this.

Gr,

Ronald van der Kooij