Duo has a habit of using the present progressive of "to go" + [infinitive] (the "phrasal future") to describe future events, instead of the actual future tense, e.g., "I am going to go" instead of "I will go". This usage is not good English idiom. Phrasal future in English most often is a current statement of an intent to do something in the future, e.g., "I am going to go into town tomorrow". It should not be used as a substitute for future tense. In this exercise, "I will be warm" is much better English idiom than "It is going to be warm".
I sometimes think that Spanish was the first language Duo presented to the public, because that language has the Phrasal Future, which uses "to go" + infinitive interchangeably with actual future tense - or at least that's what Duo teaches us. I'm now uncertain as to whether that usage even in Spanish is correct.
Nevertheless, since many of the English sentences would already have been in this form for Spanish, when Duo migrated to other languages, the developers simply brought the Spanish Phrasal Future along, instead of changing the sentences into English future tense. If that is actually true, then it was a big mistake, because in English, one should use future tense, not phrasal future, for future events.
I am an ESL teacher (qualification from Trinity College London). You have a couple of misconceptions about future tense in English. "Going to " and "will" are both future tense, but they are used differently. Neither one is classified as an idiom. It is also correct grammar to use present simple and present continous/progressive for the future in certain cases.
As I tell my students: Past tense is the most irregular, and future tense is the most complicated.
That follows standard Russian grammar. You have an adjective with the double vowel (or vowel + й) suffix. Then there are short forms that can be used predicatively with just the singular vowel (or no vowel in the masculine case) endings that resemble nouns. The neuter short form (in -о) then becomes the adverb.