remove support for Solaris (fixes #5216)

[jameslamb]

Fixes #5216 .
Related to #3513.

Per the discussion in #5217 (comment), this PR proposes removing support for the Solaris operating system in LightGBM.

That support was added only to get {lightgbm} (the R package) onto CRAN, so that it would be easier for R users to install.
There is strong evidence (provided in that comment) that CRAN no longer requires packages to be installable on Solaris.

I'm proposing removing this support for the following reasons:

1. R Hub is the only easy-to-use option we have found for testing that LightGBM is portable to Solaris, and the fact that CRAN no longer requires such portability means that incidents like [ci] Solaris R hub build is failing: "installation of package ‘Matrix’ had non-zero exit status" #5216 which prevent us from testing on R Hub could easily happen again
2. LightGBM is already struggling from a lack of maintainer activity, and removing a source of maintenance burden will allow us to focus our limited time and attention on higher-value areas of the project.

[guolinke]

I like the PR that deletes codes 😆

[StrikerRUS]

Excellent, thanks a lot! Really like to see these changes!

[StrikerRUS]

Suggested change

	, message = "Skipping on Solaris and 32-bit Windows"
	, message = "Skipping on 32-bit Windows"

[jameslamb]

Thanks for merging this @guolinke , but I didn't get a chance to address the suggestion in #5226 (comment). I'll open a follow-up PR to address that.

In the future, can you please avoid merging before all suggestions have been addressed?

[guolinke]

Thanks for merging this @guolinke , but I didn't get a chance to address the suggestion in #5226 (comment). I'll open a follow-up PR to address that.

In the future, can you please avoid merging before all suggestions have been addressed?

Sorry, I just saw 2 approves, will check the details next time.

[jameslamb]

no problem! @StrikerRUS, @jmoralez and I sometimes approve but with minor suggestions, meaning "I approve assuming you fix this minor suggestion", to avoid another round of reviews.

[guolinke]

no problem! @StrikerRUS, @jmoralez and I sometimes approve but with minor suggestions, meaning "I approve assuming you fix this minor suggestion", to avoid another round of reviews.

Actually, I tried the GitHub App on my mobile phone to review issues and codes yesterday, and didn't realize it hides the approved comment by default 🤣 .

[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.