Use proper GUID in generated rss

Describe the bug

Right now in rss field <guid> is filled with link to podcast URL, same as in <link> (or podcast source if it was imported from another hosting), but it should be proper uuid, like, for example, 4a8993ff-9a16-4475-a82a-9bb31552eb2d

Steps to reproduce

1. Create podcast and publish episodes in it
2. Inspect RSS

Expected behavior

Proper GUIDs used in <guid> fields for episodes

Actual behavior

Field <guid> is filled with url of episode

Relevant logs and/or screenshots

none

Context

• Castopod: 1.1.2
• OS: Ubuntu 22.04.1 LTS
• Web server: Apache

Possible fixes

Probably the place what created problem is this: https://code.castopod.org/adaures/castopod/-/blob/develop/app/Entities/Episode.php#L459