Accept the Disk spelling for the DISK backup source grant

GRANT READ ON Disk failed with a syntax error while GRANT READ ON DISK
worked. Source names are matched case-sensitively: AccessTypeObjects
validate() looks the name up in a std::map that holds the stringified
enumerator plus the aliases declared in APPLY_FOR_SOURCE, and DISK
declared none.

Every other source whose user-facing name differs from its enumerator
registers that name (File, PostgreSQL, AzureBlobStorage, and so on), and
the ones with no alias are initialisms already spelled in uppercase (URL,
S3, HDFS, ODBC, JDBC, NATS). DISK was the only non-initialism missing
one, even though Disk(...) is exactly how the locator is written in
BACKUP and RESTORE, so the natural spelling was the one that failed.

The alias only widens the accepted input. unifySource normalizes it, so
the grant is stored and displayed as DISK, and REVOKE READ ON DISK
removes a grant made as Disk. system.privileges is unchanged, because its
aliases column is built from the AccessType enum rather than from this
table, so no privilege reference moves.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>