Fix release licensing metadata and build documentation

[tisonkun]

The source distribution's LICENSE lists Catch2 even though CMake downloads it separately, and the NOTICE year and documented CMake minimum are stale. This follows up on the metadata and documentation issues identified while validating 5.3.0-rc1 for the next release.

• Remove the Catch2/Boost entry so LICENSE describes the bundled sources, as required by the ASF Release Policy.
• Extend the ASF copyright year in NOTICE through 2026.
• Document the actual CMake minimum, 3.16.0.

Validation: Apache RAT 0.16.1 passed on the staged Git tree via tools/rat-check.sh; git diff --cached --check passed.

[coveralls]

Coverage Report for CI Build 37173354180

Coverage increased (+0.01%) to 82.385%

Details

• Coverage increased (+0.01%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	21430
Covered Lines:	17655
Line Coverage:	82.38%
Coverage Strength:	1372975.88 hits per line

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💛 - Coveralls

[leerho]

Thank you!