[ci] Update MacOS version at Travis

[nabokovas]

There is a new version of The macOS Build Environment. So I propose changes for you to use a modern one. https://docs.travis-ci.com/user/reference/osx/

[jameslamb]

Thanks for taking the time to contribute! But I'm -1 on this change.

We do try to test LightGBM against somewhat-reason versions of software, but we also want to be sure that the library works well on widely-used configurations. XCode 12.2 is less than a month old (https://xcodereleases.com/), so I'd guess that many of our users who are using LightGBM on Mac haven't updated to it.

But I'll defer to @StrikerRUS 's opinion

[StrikerRUS]

@nabokovas Thank you very much for this update! You've just done my job 🙂

@jameslamb Let me kindly disagree with you. According to our policy, we try to use the most recent environments at Travis (#3393 (comment)) and update them quite often. I remember, we even used betas.

I believe it's better to spot a compilation problem at our CI before users come and report it.
Moreover, we are speaking about macOS where it seems users tend to use the last available software in contrast to Linux where one have to use 10-years old CentOS distribution.

FYI, yesterday I tried to use Big Sur, but unfortunately without any luck due to dependency unavailability: #3499.

[jameslamb]

ah I see, ok then I am ok with this change.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.