Remove duplicate browserlist line in v6 upgrade 📖

[sunny]

Both steps 5 and 10 of the v6 upgrade guide asked to remove .browserslistrc. This PR keeps only the last one.

Also:

• reformatted list a bit (spacing and indented code blocks)
• mention that the bin/yarn to create must be executable
• added assets:precompile as a final check
• added assets:clobber reminder

[guillaumebriday]

Thanks @sunny 🙏