optimize bitops

[bitfaster]

Bring forward changes from https://github.com/bitfaster/BitFaster.Caching/pull/667/changes.

Original code had a clause specifically for .NET3.1 that had different behavior (see code below). This build target will be removed in the next major release, therefore remove now rather than fixing it.

        public static uint CeilingPowerOfTwo(uint x)
        {
#if NETSTANDARD2_0
            // https://graphics.stanford.edu/~seander/bithacks.html#RoundUpPowerOf2
            --x;
            x |= x >> 1;
            x |= x >> 2;
            x |= x >> 4;
            x |= x >> 8;
            x |= x >> 16;
            return x + 1;
#elif NET6_0_OR_GREATER
            return BitOperations.RoundUpToPowerOf2(x);
#else
            return 1u << -BitOperations.LeadingZeroCount(x - 1);
#endif
      }

[coveralls]

coverage: 99.212% (+0.001%) from 99.211% — users/alexpeck/bitops into main