Error in square root for cpp_dec_float_100

I recently updated boost from 1.71.0 to 1.80.0 and I started getting an error. After some investigation the problem arises from an incorrect rounding on the square root of 49 that makes it 7.000...01 instead of just 7. Here's the code that shows the issue

```c++
#include
#include

using boost::multiprecision::cpp_dec_float_100;
using std::string;

int main() {
cpp_dec_float_100 n(49);
n = sqrt(n);
if (n == (uint32_t) n)
std::cout << "This should be logged as sqrt(49) = 7" << std::endl;
else
std::cout << "Error: sqrt(49) != 7" << std::endl;

std::cout << std::endl << n.str() << std::endl;

return 0;
}
```

```
Error: sqrt(49) != 7

7.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001
```

I also replicated the issue [here](https://wandbox.org/permlink/LdTnsrcG6DPbavgB) with version 1.79.0. In version 1.71.0 the calculation was correct.

Start with the provided reproduction and the sqrt path exposed by boost/multiprecision/cpp_dec_float.hpp, comparing behavior with Boost 1.71.0 and 1.79.0/1.80.0. Trace why sqrt(49) produces a nonzero final digit; done means the reproduction compares equal to 7 and prints the expected value.

Written by the indexing model from the issue text.