build-configs.yaml: move rust-next to Rust 1.68

This moves both `rust-for-linux_rust-next` and `next`'s Rust variant.

As discussed [1], for this kind of upgrades, we will move KernelCI first
and then I will apply push the branch. The run is expected to fail if
it happens to run between those two moves.

Link: kernelci#1890 (comment) [1]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

config/core/build-configs.yaml

@@ -1189,8 +1189,8 @@ build_configs:
         architectures:
           <<: *arch_clang_configs
 
-      rustc-1.62:
-        build_environment: rustc-1.62
+      rustc-1.68:
+        build_environment: rustc-1.68
         fragments: [rust, rust-samples, kselftest]
         architectures:
           x86_64:
@@ -1350,8 +1350,8 @@ build_configs:
     tree: rust-for-linux
     branch: 'rust-next'
     variants:
-      rustc-1.62:
-        build_environment: rustc-1.62
+      rustc-1.68:
+        build_environment: rustc-1.68
         fragments: [rust, rust-samples, kselftest]
         architectures:
           x86_64: