docs(BUILD.md): require GCC 11 or higher (#4700)

Update minimum compiler requirement for building the codebase. The
feature "using enum" is required. This feature was introduced in C++20.

Updating the C++ compiler to version 11 or later fixes this error:

```
Building CXX object CMakeFiles/xrpl_core.dir/src/ripple/protocol/impl/STAmount.cpp.o
/build/ripple/binary/src/ripple/protocol/impl/STAmount.cpp: In lambda function:
/build/ripple/binary/src/ripple/protocol/impl/STAmount.cpp:1577:15: error: expected nested-name-specifier before 'enum'
 1577 |         using enum Number::rounding_mode;
      |               ^~~~
```

Fix #4693

BUILD.md

@@ -44,7 +44,7 @@ The [minimum compiler versions][2] required are:
 
 | Compiler    | Version |
 |-------------|---------|
-| GCC         | 10      |
+| GCC         | 11      |
 | Clang       | 13      |
 | Apple Clang | 13.1.6  |
 | MSVC        | 19.23   |