Requirement R8 - Migrate shared database password from plaintext to credential manager

[pat672]

As a jabref user
I want my shared database password to be migrated from plaintext to the credential manager store
So that my password doesn't remain unsecured on the system despite the credential manager feature being active

When users upgrade from a jabref version where their password was stored in plaintext to a version which stores it using a credential manager, we must make sure that the previously stored plaintext password is copied over to the credential manager store and removed from the plaintext data store. Otherwise the password will remain unsecured, without the user being aware of this fact.

[pontuscm]

Tested manually by building branch main, storing password, building branch "r/7-8" with this migration implementation. Then opening shared database connection (which contains the password from stored plaintext in previous version) and connecting to database. The password then pops up in the OSX credentials manager.