Update the MR template for the RISC-V crew

Amends the MR template for RISC-V.

• Removes Thomas P as a default assignee and reviewer.
  • He no longer reviews.
• Prefill the "Manual Testing" section with a good default.
• Add a "Benchmark" section to encourage authors to validate their MR's performance impact.
  • This is an attempt to build more awareness around performance-impacting changes in the absence of an automated test.
• Add a "Tasks for the Author" section that authors are encouraged to tick before giving it to reviewers.
  • I want to improve the MR quality by building a habit around these tasks. Otherwise, the burden is always on the reviewers to point out these lint.