Fix CI test matrix inconsistencies

[mr-raj12]

• Align PR integration Borg version from 1.4.2 to 1.4.3
• Remove stale exclude rule for Borg 2.0.0b12 / Python 3.8
• Update build-macos.yml default from macos-14 to macos-15

Description

Fix three configuration inconsistencies in the CI matrix generation script
and macOS build workflow:

File	Issue	Change
.github/scripts/generate-matrix.sh	PR integration tests use Borg
1.4.2 while master uses 1.4.3	1.4.2 → 1.4.3
.github/scripts/generate-matrix.sh	Stale exclude rule for Borg
2.0.0b12 / Python 3.8 (neither in matrix)	Removed
.github/workflows/build-macos.yml	Build defaults to macos-14 while
test matrix uses macos-15	macos-14 → macos-15

Related Issue

Closes #2377

Motivation and Context

• A PR can pass CI testing against Borg 1.4.2 but fail after merge when
  master runs 1.4.3
• macOS releases are built on macos-14 while tests run on macos-15, masking
  potential environment issues
• The stale exclude rule references Borg 2.0.0b12 and Python 3.8, neither
  of which exist in the current matrix, adding dead configuration

How Has This Been Tested?

• CI-only configuration changes with no functional code impact
• JSON output of both matrix script paths validated locally (all 4 blocks
  produce valid JSON, no stale excludes, consistent Borg versions)
• YAML syntax of build-macos.yml validated locally
• The PR's own CI run will confirm the updated matrix works end-to-end

Screenshots (if appropriate):

N/A — No visual changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing
  functionality to change)

Checklist:

• I have read the
  CONTRIBUTING guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the Developer
Certificate of Origin.