Fix CI test matrix inconsistencies

[mr-raj12]

• Align PR integration Borg version from 1.4.2 to 1.4.3
• Update build-macos.yml default from macos-14 to macos-15

Description

Fix two 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/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

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, 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.

[ThomasWaldmann]

LGTM.

Needs a rebase onto current master.

[m3nu]

Is this overlapping with another PR you did?

[mr-raj12]

#2384 already took care of the stale exclude rule. I've rebased and dropped that bit, so this is just the two leftover fixes now: borg version alignment in PR integration tests (1.4.2 → 1.4.3) and the macos build default (macos-14 → macos-15). Updated the PR description too.

[ThomasWaldmann]

LGTM!