Fix CI test matrix inconsistencies (Borg version, stale exclude, macOS mismatch)

[mr-raj12]

The problem

The CI matrix generation script (.github/scripts/generate-matrix.sh) contains multiple configuration inconsistencies that can cause unreliable CI results and environment mismatches between testing and release builds.

Specifically:

• Borg version mismatch: PR integration tests use Borg 1.4.2 (line 28) while the master branch uses 1.4.3 (line 8). This can allow a PR to pass CI but fail after merging.
• Stale exclude rule: Line 15 contains an exclude entry for borg-version: 2.0.0b12 and python-version: 3.8, neither of which exist in the current matrix. This results in dead configuration.
• macOS version mismatch: build-macos.yml defaults to macos-14 (line 16), while the test matrix uses macos-15 (line 7), meaning releases are built on a different OS than what is tested.

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Requested Solution

• Align PR integration Borg version to 1.4.3 in generate-matrix.sh (line 28).
• Remove the stale exclude rule from generate-matrix.sh (line 15).
• Update the default macOS version in build-macos.yml to macos-15 (line 16).

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Alternatives

None — these are straightforward consistency and cleanup fixes.

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Additional context

Affected files:

• .github/scripts/generate-matrix.sh (lines 8, 15, 28)
• .github/workflows/build-macos.yml (line 16)