feat: upgrade remaining templates to Bootstrap 5

[jrokeach]

The following templates were not upgraded during the Bootstrap 4 → 5 migration and still use removed/renamed classes:

• previous-events.html: .embed-responsive.embed-responsive-16by9 → .ratio.ratio-16x9; .font-weight-bold → .fw-bold; .font-italic → .fst-italic; .text-left → .text-start
• general-rules.html: .card-deck → .row.row-cols-*
• member-rules.html: .card-deck → .row.row-cols-*
• next-event.html: .text-left → .text-start; .text-center → .text-center (unchanged); .card-group → .row

These pages will render with broken layout until fixed.