[docs] Bump minimum GCC version to 7.4

GCC 7.3 cannot build 16.x releases.
```
In file included from /tmp/llvm-16/llvm/lib/Transforms/IPO/AttributorAttributes.cpp:14:0:
/tmp/llvm-16/llvm/include/llvm/Transforms/IPO/Attributor.h:1137:32: error: duplicate initialization of ‘llvm::AnalysisGetter::HasLegacyWrapper<Analysis, std::void_t<typename Analysis::Lega
cyWrapper> >’
 constexpr bool AnalysisGetter::HasLegacyWrapper<
                                ^~~~~~~~~~~~~~~~~
       Analysis, std::void_t<typename Analysis::LegacyWrapper>> = true;
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/llvm-16/llvm/include/llvm/Transforms/IPO/Attributor.h:1137:32: error: got 1 template parameters for ‘constexpr const bool llvm::AnalysisGetter::HasLegacyWrapper< <template-parameter-1
-1>, <template-parameter-1-2> >’
/tmp/llvm-16/llvm/include/llvm/Transforms/IPO/Attributor.h:1137:32: error:   but 2 required
```

The 17.x and main branches have more failures, e.g.

```
llvm/lib/Transforms/Vectoriz...