Replace BIP 77 mermaid diagrams with ascii diagrams

Updated sequence diagrams to use text format instead of mermaid syntax responding to @achow101's comment

I cargo cult'd the RFC Rules:

“How are images handled for the plain text version of an RFC?” The RFC Editor will accept both ASCII art and SVG. If only ASCII art is provided, it will be included in all publication formats. If ASCII art and SVG are both provided, ASCII art will be included in the plain text, and SVG in all other outputs. A note indicating alternative artwork is available is strongly advised. If only SVG is provided, a URI will be included in the plain-text publication format pointing to the HTML version. All artwork and figures should have a complete written description to support assisted reader technology.

see: https://www.rfc-editor.org/rse/format-faq/

Since BIPs don't publish html/pdf renders, ASCII art seems like the right choice to render everywhere. Since normative prose is already provided, I chose not to include a written description of the diagrams to support assisted reader tech.

cc @nothingmuch