remove support for v0.20

• search for all v0.20

test on the following spack versions:

• branch develop -> 0.24
• branch releases/v0.20 -> raise exception
• branch releases/v0.21 -> 0.21
• branch releases/v0.22 -> 0.22
• branch releases/v0.23 -> 0.23
• tag v0.22.3 -> v0.22
• random recent commit + --develop -> v0.24

remove develop as an option that is passed into makefiles