Retire mambaforge - addendum to 3774

[valeriupredoi]

Description

A few more refrences to Mambaforge/mambaforge that I missed in #3774 - was in a hurry get that over the fence so we get our tests back, @bettina-gier pointed to me we have a few more changes to make :beer:

Note the RTD configuration is currently unable to get the Miniforge3 variant, see https://github.com/ESMValGroup/ESMValCore/issues/2557

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[valeriupredoi]

yay! Many thanks @bettina-gier :beer: