fix(timepicker): use a supported Bootstrap meridian button style

[mnkprs]

The AM/PM button uses btn-default, which Bootstrap 4 and 5 do not define. Switch it to btn-secondary, as suggested in the issue, so the button gets its background, border, hover and focus styles.

Fixes #6830.

Validation:

• npm test -- --parallel=2 --runInBand: 1,548 passed, 137 skipped across 24 projects.
• npx nx build timepicker --configuration=production: passed.
• npm run lint -- --parallel=2: passed for all 46 projects.
• Checked the template's button markup in Chrome with the bundled Bootstrap 4.5.3 and 5.2.3 stylesheets. Background, border, hover and focus styles now apply in both versions.

PR checklist:

• Read and followed the contributing guide.
• Built and tested locally.
• Ran the existing Timepicker tests and checked the changed styling in Chrome.
• No API documentation or demo changes needed.