Release calendar updated

[Mark Waite]

I've placed Jenkins 2.516.1 through 2.516.3 on the Jenkins events calendar and have inserted placeholders for the next LTS after that.  I think that we should plan for the two week break at the end of December as noted in the list below

Dates include:

• 2.516.1-rc - July 9, 2025
• 2.516.1 - July 23, 2025
• 2.516.2-rc - August 6, 2025
• 2.516.2 - August 20, 2025
• 2.516.3-rc - September 3, 2025
• 2.516.3 - September 17, 2025
• ...
  
• 2.5xx.3 - December 10, 2025
• Two week break - December 22, 2025 - January 2, 2026
• 2.5yy.1-rc - January 7, 2026
• 2 .5yy.1 - January 21, 2026

Others are welcome to propose calendar changes or corrections.

Thanks,

Mark Waite