Upgrade Rust to 1.68.0

[Zixing Liu]

Upgrade Rust to 1.68.0.

Notable changes from upstream:

- Cargo's sparse protocol: No longer need to wait through `git clone` for the crates index updates!
- Local Pin construction: More advanced version of `tokio::pin!` macro.
- Default alloc error handler: Now memory allocation failures no longer cause an abort()... only in the `no_std` builds though.

[Andrea Righi (arighi)]

+1 on this, apparently we will need Rust 1.68 to provide the Rust support in the kernel with linux 6.5 in mantic (https://<email address hidden>/T/)

[Launchpad Janitor (janitor)]

This bug was fixed in the package rustc - 1.68.0+dfsg0ubuntu1-0ubuntu1

---------------
rustc (1.68.0+dfsg0ubuntu1-0ubuntu1) mantic; urgency=medium

  * New upstream release 1.68.0 (LP: #2009920)
    - d/copyright: Update copyright information for new dependencies
      and also remove copyright data for deleted dependencies
    - d/patches: Refresh patches
    - d/control: Update vendored deps field
    - d/control: Refresh vendored dependencies list
  * d/p/ubuntu-rustdoc-replace-cargo-links.patch: Replace Cargo related
    links with external web links instead of the local links to avoid
    link-checker errors (because we package Cargo separately)
  * Merge Cargo source package into Rust (LP: #2020000)
    - d/*: copies Cargo Debian manifests into Rust
    - cargo: add Cargo sources to the source tree
    - d/patches: adapt Cargo-specific patches to the new source tree
      structure
    - d/patches, vendor: remove vendored C sources and prebuilt blobs
    - vendor: vendor Cargo-specific dependencies
    - d/control: update vendored dependency list
    - d/rules: revise openssl source check
    - d/cargo.*: install Cargo-specific files
    - d/config: enable Cargo module in the rustbuild configuration
    - d/patches: remove non-Linux platform modules
    - d/rules: use system libssh2 for Cargo
    - d/patches: restore the usage of rustc-workspace-hack to
      reduce the build time and avoid crates feature resolution failure
    - d/patches: remove patches that patch out Cargo from the docs
    - d/rules, d/cargo.*: build and install Cargo API docs
    - d/control, d/rules: install Cargo bash completions
    - d/copyright: update copyright information
    - d/control: make cargo-doc recommends rust-doc

 -- Zixing Liu <email address hidden> Wed, 17 May 2023 14:15:17 -0700