Update download count and signing guide in readme

Revised the dbatools download count to 7 million and removed mention of automation scripts from the signing migration guide reference for clarity.

Bump module and dependency versions

Updated dbatools.psd1 to version 2.5.1 and increased the required dbatools.library module version to 2025.7.20. Also updated dbatools-index.json binary, likely reflecting related changes.

oops, forgot the version update