std.builtin.CompilerBackend: useless renaming

[BratishkaErik]

Remove stage1 tag since old C++ compiler was removed NUKED in #13560, rename remaining stage2_* backends to zsf_* (choosed this prefix for clarity over simply omitting old prefix, but if you want I can omit it or change to other prefix).
Gains one more free tag (out of 18_446_744_073_709_551_615) and one possible bug removed (switching/checking if (backend == .stage1) even when this compiler was removed) (I forgot that this enum is non-exhaustive)

[mlugg]

I think you're misunderstanding the purpose of this enum. The whole point of it is to work around issues in compiler implementations, using compiler-specific logic. Because the stage1 compiler existed at one point in time, the enum tag exists to allow programs - both old and new - to support it if they desire.

IMO, this enum should never have tags removed: it is intended to be a record of every major Zig compiler implementation which has existed, and removing tags is a completely pointless breaking change with no benefits and some potential (if minor) downsides.

Additionally, renaming stage2_* to zsf_* could lead to confusion: this isn't the only compiler implementation the ZSF have ever worked on (because stage1 also existed). This change, again, has zero benefits and a potential downside of confusion.

[BratishkaErik]

Ok, nearly got it, but should it then be commented as such (/// All tags are preserved for historical and compatibility purposes.)? I didn't find something similar here and thought that it's fine to free one tag while we are pre-1.0.