[Version-unspecific] Fix migration test version assertions

The store migration runner advances through the active network minor version. Three migration tests hardcode a final store version of 135, causing them to fail after a bump to 1.136.0 despite successful migration.

Update four assertions in the ZEC signing-height migration tests and affiliate-collector rollback test to expect mgr.GetVersion().Minor.

Validation:

• Reproduced all three original failures at 1.136.0.
• Updated tests pass at both 1.135.0 and 1.136.0.
• Configured lint, gofumpt, and diff checks pass.