Bump "Modern Compiler" requirements, to set 0.8.0 as minimum "reasonable" version

[chaals]

v1 of the spec sets an earliest acceptable compiler of v0.3.0 because there are no reliable security assessments of older compilers. v2 added a requirement to use a compiler newer than 0.6.0 as a way to cirucumvent a lot of compiler bugs, that meant we could refer to those from v1 for the case where someone was still using an outdated compiler.

At the time we set a possible expectation that we would repeat this process for v3 moving to a higher minimum.

Should we bump the "minimum reasonable compiler" again (while making it possible, with more work, to conform even while using something outdated like 0.3.x)? If so, to what?

0.8.0 predates the v1 spec release, and has been around for about 5 years - somewhere around half the lifespan of Ethereum...