Update schema generator for Betterleaks

[wesley-dean]

Fixes #8801

REPOSITORY_BETTERLEAKS_PR_COMMITS_SCAN,
REPOSITORY_BETTERLEAKS_PR_SOURCE_SHA,
and REPOSITORY_BETTERLEAKS_PR_TARGET_SHA are supported by Betterleaks and
documented from its descriptor, but are absent from the generated MegaLinter
configuration JSON Schema. As a result, schema validators such as v8r reject
otherwise valid .mega-linter.yml configurations.

This change narrowly adds those three Betterleaks-specific variables to the
generated configuration schema. It does not change Betterleaks runtime behavior
or the descriptor format.

Proposed Changes

1. Update the automation script that builds the JSON schema to include the three missing entries for REPOSITORY_BETTERLEAKS
2. Update changelog

Readiness Checklist

Author/Contributor

• Add entry to the CHANGELOG listing the change and linking to the corresponding issue (if appropriate)
• If documentation is needed for this change, has that been included in this pull request

Reviewing Maintainer

• Label as breaking if this is a large fundamental change
• Label as either automation, bug, documentation, enhancement, infrastructure, or performance

[nvuillam]

Thanks Wes :)