Rename tokenExpiry field to prevent Folders corruption issues when upgrading from old versions of this plugin

[dwnusbaum]

XStream attempts to deserialize all fields in XML files that have a corresponding field in the class, even if it is marked transient. This means that #325 did not actually fix the issues described in #323, #324, and #326, which were caused by #223.

This should fix #323, #324, and #326. I tested it manually with a corrupted folder.

Testing done

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