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 ESMValGroup/ESMValCore#2398), which causes an error in the diagnostic austral_jet/main.ncl. This PR fixes that.

• Addresses parts of Recipe test results for ESMValCore v2.11.0rc1 ESMValCore#2421 (comment)

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[valeriupredoi]

And here too (DBPR 😁)