Stabilize feature uint_bit_width

[sorairolake]

Tracking issue: #142326
FCP is finished in #142326 (comment)

Closes #142326

@rustbot modify labels: +T-libs-api

[rustbot]

r? @scottmcm

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[sorairolake]

Cc @rust-lang/wg-const-eval

[rustbot]

Failed to set assignee to Mark-Simulacrum: error sending request

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[sorairolake]

r? libs-api