[openstreetmap/openstreetmap-website] Outdated versions in INSTALL.md cause errors (#2909)

[nlaratta]

When following the instructions listed in INSTALL.md, which uses Ubuntu 18.04 and Ruby 2.5, we get these errors:
![Ubuntu18Errors](https://user-images.githubusercontent.com/63064624/97200565-fe012600-1776-11eb-948b-3440e5f77dfd.png)

Further explanation from a team member: "All the errors I get are in relation to this app/views/browse/note.html.erb line 41 syntax error unexpected ']'.  The brackets have a [1...] inside, if I edit the note.html.erb file so that the brackets on line 41 have [1..2] within them, the error goes away."

When following the instructions using Ubuntu 20.04 with Ruby 2.7, we get no errors. We asked in IRC-dev if this means that INSTALL.md might be due for an update, or if we messed up. We were told by a few people that Ubuntu 20.04 LTS is the version used by the OSMF Servers. This goes against what's stated in the document.

Is INSTALL.md due for an update?

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