Improve issue template and optimize schedule

[janhohenheim]

• Remove duplicate title
• Fix outdated lint information
• Remove TODOs
• Add reminders for which places to post the monthly updates on (All of the Discord server's moderation teams have agreed to get these)
• Assign me to some things
• Merge monthly calls for submissions with spreading the released newsletter
• Move the generation of the next newsletter to the publication date
• Adopt the freeze period schedule proposed by @Vrixyz in https://github.com/rust-gamedev/rust-gamedev.github.io/issues/1417#issuecomment-1764534286

From the linked issue, the schedule is as follows:

timeline
    title This Month in Rust Gamedev News
    3rd : Release previous news entry!
        : (Release) social media posts, 
        : (Release) links to discussions
        : Add a draft of next month's newsletter
    4th-27th : Contributor creates an issue
        : Community/editors review
        : Merge into source
    28th : PR freeze<br> Expect any pending PRs to be reported.
         : Last touches<br>global review pass

The only difference here is is that I changed the 1st to the 3rd to have some more time for editing.