Align package product_name with the major-stream product label

[atomicturtle]

Summary

• packages[].product_name was Rocky Linux 9 aarch64 while affected_products[].name was Rocky Linux 9.6 aarch64 (distro-tools#71). Point-release mirror names froze the minor that was current at clone time, so 9.7 scanners dropped 9.6-labeled fixes.
• Collapse {major}.{minor} in the display name used for both fields, refresh stale product_name on rematch, and default generate_rocky_config.py mirror names to the major (Rocky Linux 9 aarch64 from --version 9.6). riscv64 labels without a minor are unchanged.

Existing rows pick up the stream label on the next rematch. This does not invent empty-package RLSAs.

Test plan

• python -m unittest apollo.tests.test_rh_matcher_activities.TestStreamProductName
• After rematch, curl -s https://apollo.build.resf.org/api/v3/advisories/RLBA-2025:2973 | jq '{products: [.affected_products[].name], pkgs: [.packages[].product_name] | unique}' — both sides are Rocky Linux 9 <arch>, not 9.6.

Closes #71

[rockythorn]

Review: Approve

Tests pass, no merge conflicts with main.

Verified

• stream_product_name() correctly collapses Rocky Linux 9.6 aarch64 → Rocky Linux 9 aarch64 when match_minor_version is set
• Names without a minor version (e.g., Rocky Linux 10 riscv64) are unchanged
• Zero minor version (10.0) correctly collapses to 10
• generate_rocky_config.py updated to default mirror names to major version when --version includes a dot
• Stale product_name on existing packages is updated on rematch (not just new packages)
• bazel test //apollo/tests:test_rh_matcher_activities passes

Note for merge ordering

This PR, along with #89, #90, and #91, all modify rh_matcher_activities.py. They'll need sequential rebasing after the first one merges. This PR is the most independent of the four — it can merge in any position.

Since #89 needs a migration, #90 needs a BUILD fix, and #91 needs a Python 3.9 compat fix, this PR is the most merge-ready of the chain.