Update crossbeam-utils requirement from =0.8.16 to =0.8.21

[dependabot]

Updates the requirements on crossbeam-utils to permit the latest version.

Release notes

Sourced from crossbeam-utils's releases.

crossbeam-utils 0.8.21

• Improve implementation of CachePadded. (#1152)

Changelog

Sourced from crossbeam-utils's changelog.

Version 0.8.4

• Remove dependency on cfg-if. (#1072)

Version 0.8.3

• Bump the minimum supported Rust version to 1.61. (#1037)

Version 0.8.2

• Bump the minimum supported Rust version to 1.38. (#877)

Version 0.8.1

• Support targets that do not have atomic CAS on stable Rust (#698)

Version 0.8.0

• Bump the minimum supported Rust version to 1.36.
• Bump crossbeam-channel to 0.5.
• Bump crossbeam-deque to 0.8.
• Bump crossbeam-epoch to 0.9.
• Bump crossbeam-queue to 0.3.
• Bump crossbeam-utils to 0.8.

Version 0.7.3

• Fix breakage with nightly feature due to rust-lang/rust#65214.
• Bump crossbeam-channel to 0.4.
• Bump crossbeam-epoch to 0.8.
• Bump crossbeam-queue to 0.2.
• Bump crossbeam-utils to 0.7.

Version 0.7.2

• Bump crossbeam-channel to 0.3.9.
• Bump crossbeam-epoch to 0.7.2.
• Bump crossbeam-utils to 0.6.6.

Version 0.7.1

• Bump crossbeam-utils to 0.6.5.

Version 0.7.0

• Remove ArcCell, MsQueue, and TreiberStack.
• Change the interface of ShardedLock to match RwLock.
• Add SegQueue::len().
• Rename SegQueue::try_pop() to SegQueue::pop().
• Change the return type of SegQueue::pop() to Result.

... (truncated)

Commits

• ccd83ac Prepare for the next release
• 54988eb Calculate layout in const context
• 761d0b6 Port #1146 & #1147 to deque::Injector and queue::SegQueue
• 8144fbb Remove optimistic spinning from Context::wait_until
• a92f6c4 Bump peter-evans/create-pull-request from 5 to 7 (#1153)
• 66d41a9 channel: Add new_biased constructor for biased channel selection (#1150)
• d0d0a80 CachePadded: Use 128-byte alignment on arm64ec
• f757eef Add comment about fixed rustc bug
• 71d8bb0 ci: Update minimum tested Rust version to 1.63
• 5092a9b ci: Clean up no_atomic.sh
• Additional commits viewable in compare view

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[dependabot]

Looks like crossbeam-utils is no longer being updated by Dependabot, so this is no longer needed.