Do not introspect exclusion constraints as Gist indexes

[janpio]

#15173 showed that we currently introspect exclusion constraints incorrectly as Gist indexes.

Until we add explicit support for them via #17514, we should at leat not introspect them as indexes but just skip them and maybe output a warning to the user on how to handle this unsupported type of constraint in the meantime.

[jkomyno]

Note:

• if EXCLUDE has a WHERE clause, it’s not introspected
• if EXCLUDE doesn’t have a WHERE clause but refers to columns via immutable functions, it’s not introspected
• if EXCLUDE doesn’t have a WHERE clause and refers to columns directly, regardless of whether their type is supported by Prisma, it’s introspected

This is true for both prisma@4.3.1 and prisma@4.12.0.
See Slack thread.