Release CBMC 6.9.0

[tautschnig]

This release ensures cross-platform determinism of formulae created by CBMC (for a given input program), ensuring consistent solver performance (via #8830, #8952). This releases fixes a variety of bugs, including the problem of orphan (SMT-solver) child processes left behind after terminating CBMC (via #8900), and various encoding improvements for unions (via #8950, #8958) and multi-dimensional arrays (via #8705).

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[Copilot]

Pull request overview

Release prep PR updating CBMC to 6.9.0 and adding corresponding release notes.

Changes:

• Bump Rust crate version to 6.9.0.
• Bump build/config version to 6.9.0.
• Add CHANGELOG entry for CBMC 6.9.0 (summary + list of merged PRs).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/libcprover-rust/Cargo.toml	Updates libcprover_rust crate version to 6.9.0.
src/config.inc	Updates CBMC_VERSION used by build/version generation to 6.9.0.
CHANGELOG	Adds the CBMC 6.9.0 release notes section and compare link.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.50%. Comparing base (2f62eea) to head (f884c37).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8961   +/-   ##
========================================
  Coverage    80.49%   80.50%           
========================================
  Files         1704     1704           
  Lines       188805   188805           
  Branches        73       73           
========================================
+ Hits        151986   151989    +3     
+ Misses       36819    36816    -3     

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