[docs] Bump minimum GCC version to 7.4

[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/Vectorize/LoopVectorize.cpp: `error: cannot decompose class type ‘std::pair<llvm::Value*, const llvm::SCEV*>’: ...`
```

We probably should just give up 7.1 and say that GCC<=7.3 is unsupported.
There is evidence that GCC 7.4 works.
I have verified that GCC 7.5 is able to build `check-{llvm,clang,clang-tools,lldb,lld,polly,mlir,bolt}`,
but not flang due to at least `flang/Common/enum-class.h` and a `<charconv`> in a unittest.

Link: https://discourse.llvm.org/t/require-gcc-7-5-as-gcc-7-3-cannot-build-llvm/72310
Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D156286

(cherry picked from commit 1e06b82bded69fe627d6cd62ecff236fca15f39b)