Auto merge of #152721 - ehuss:beta-revert-target-spec, r=jackh726

[beta] Revert destabilise target-spec-json

This reverts https://github.com/rust-lang/rust/pull/150151 in order to deal with https://github.com/rust-lang/rust/issues/151729 where the destabilization caused a problem with building rustc itself with JSON target specs. There's a fix at https://github.com/rust-lang/rust/pull/152677, but we would prefer to not backport that, and instead give ourselves more time to work out the kinks. Also, the destabilization was incomplete, and the rest of the changes are in 1.95 (https://github.com/rust-lang/rust/pull/151534 and https://github.com/rust-lang/cargo/pull/16557), so it would be nice to keep all the changes together in one release.

This reverts commit a89683dd954be39f433b4e0ff6380d845e80ec6b, reversing changes made to 2f1bd3f3781c90a8447e37d65a898442b8618895.