[STAR-2958] Apply changes from latest company-wide sustainability review

• Serenity's monthly budget cap is now 208h.
• Falcon's sustainability target is now 18%.
• Bebop's sustainability target is still 26%.

Also applies the following drive-by updates:

• Remove obsolete note about estimation session being closed on Day 10 of the sprint (follows up on !495 (merged)).
• Reassign Marketing Specialist role (@gabrieldamours passed it on to @cassiez).
• Link to current version of Serenity's cell-level roadmap.
• Fix link formatting in epic description template.
• Remove remaining mentions of "self-hosted" instances (follows up on !496 (merged)).

Reviewers

• @antoviaque