Release 6.1.2

[ppeble]

Bumps VERSION.txt to 6.1.2 and adds the matching CHANGELOG.md entry to trigger an automated release.

Patch release covering the fixes and changes merged since 6.1.1:

• Fix nz Canterbury Anniversary Day to the second Friday after the first Tuesday of November (holidays#285)
• Fix lu Whit Monday to fall on Monday (Easter Sunday + 50 days) instead of Sunday (holidays#289)
• Replace nil-returning year-gate functions with year_ranges
• Clarify that formal holidays are what is commonly called statutory holidays

[ppeble]

Superseded by the 7.0.0 release PR (dropping EOL Ruby 3.2 makes this a major bump).