Update Rust dependencies

[tjkirch]

Description of changes:

tools: cargo update
tools: update nonzero_ext to 0.3

Update serde-plain to 1.0
    The update_metadata crate is shared between the sources/ and tools/
    workspaces.  This update covers both so the tools/ workspace doesn't
    need to be updated again after sources/.

sources: cargo update
sources: update serde-xml-rs

Not updated: gptman, hyper (in sources/), x509-parser

Testing done:

Unit tests pass. Built AWS k8s 1.21 and ECS AMIs for x86_64 and aarch64. Confirmed systemctl status running, dmesg and journal OK. Ran a pod/task OK. Made API changes OK.

Confirmed that cargo make repo and cargo make ssm still work OK.

Built vmware-k8s-1.21 and confirmed that XML in an OVF still provides user data to it correctly, with the serde-xml-rs update.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[tjkirch]

^ Add a commit that locks to httptest 0.15.2 for now, since 0.15.3's IPv6 support doesn't yet work in CI.

[tjkirch]

I'll rebase this onto #1751 when it's done so we don't need the httptest locking.

[tjkirch]

^ Rebase on develop to get #1751 so I could remove the commit locking httptest.