v2.6.8: Fix L72 fwet for dust

This release fixes an issue with the fwet values for dust. In #376 the fwet values for dust were set to the L181 values, but unlike Ch_DU for ginoux, these old L72 values were not retained in a commented statement. This PR 'brings back' the old L72 values which should be used when gcm_setup makes an L72 experiment.

This will be non-zero-diff for L72 runs, but zero-diff for non-L72 runs.

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Fixed

• Restore L72 fwet value for dust to 0.8 DU2G_instance_DU.rc. The 1.0 values were for
  L181 and were mistakenly applied to L72. gcm_setup will select the L72 values when
  a 72-level experiment is created. This is non-zero-diff for L72 runs, but zero-diff for non-L72 runs.

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What's Changed

• v2.6: Restore L72 Dust fwet by @mathomp4 in #485
• GitFlow: Merge release/v2.6 into main for 2.6.8 release by @mathomp4 in #488

Full Changelog: v2.6.7...v2.6.8