Fixed attribute handling in austral_jet/main.ncl for iris>=3.8

[schlunma]

Description

Since version 3.8, iris distinguishes explicitly between local and global attributes (see https://github.com/ESMValGroup/ESMValCore/pull/2398), which causes an error in the diagnostic austral_jet/main.ncl. This PR fixes that.

• Addresses parts of https://github.com/ESMValGroup/ESMValCore/issues/2421#issuecomment-2115103294

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