build: Set CMAKE_OSX_DEPLOYMENT_TARGET to 26.0

[mathbunnyru]

High Level Overview of Change

This is needed to use std::from/to_chars inside an macOS environment where auto-detection thinks we're targeting lower macOS version (inside Nix).

It would also allow us to explicitly see that using a new feature requires us upping the requirement for macOS version.
So, we won't accidentally bump the requirement for macOS version again.

Context of Change

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.3%. Comparing base (c7adb21) to head (348e716).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #7785     +/-   ##
=========================================
- Coverage     82.3%   82.3%   -0.0%     
=========================================
  Files         1024    1024             
  Lines        78416   78416             
  Branches      8932    8932             
=========================================
- Hits         64550   64545      -5     
- Misses       13857   13862      +5     
  Partials         9       9             

see 4 files with indirect coverage changes

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[godexsoft]

I can see how this may upset some people

[ximinez]

Whether this is a better solution than just reverting std::from/to_chars is still up for debate, but I can at least say that this PR works as advertised.

[Copilot]

Pull request overview

Sets a default macOS deployment target in the top-level CMake configuration (and documents it) so builds don’t silently target an older macOS version—particularly in Nix-based macOS environments—when using newer standard library features.

Changes:

• Default CMAKE_OSX_DEPLOYMENT_TARGET to 26.0 (only when not already provided) before project() runs.
• Update BUILD.md to document the minimum supported macOS version and that the build defaults the deployment target.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
CMakeLists.txt	Adds a default cached CMAKE_OSX_DEPLOYMENT_TARGET for macOS builds prior to project().
BUILD.md	Documents the minimum supported macOS version and the new CMake default behavior.

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