v0.21.9 preparation

[cpu]

Description

This PR targets the rel-0.21 feature branch, preparing a 0.21.9 release. It backports #1582 to fix build issues with downstream crates that were using the read_buf feature with Rust nightly (e.g. rustls-ffi).

Proposed Release Notes

• Fixes using the (non-default) read_buf feature with Rust nightly newer than nightly-2023-11-01 by opting in to the core_io_borrowed_buf feature and updated BorrowedBuf, BorrowedCursor types.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (c34477a) 95.81% compared to head (25e6df5) 95.81%.

Files	Patch %	Lines
rustls/src/server/server_conn.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           rel-0.21    #1592   +/-   ##
=========================================
  Coverage     95.81%   95.81%           
=========================================
  Files            61       61           
  Lines         14851    14852    +1     
=========================================
+ Hits          14229    14230    +1     
  Misses          622      622           

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[cpu]

• Pushed v/0.21.9 tag
• Published rustls v0.21.9 at registry crates-io
• Created 0.21.9 GitHub release