Spring Simplify conversion of DataBuffer to String

This PR is similar to https://github.com/spring-projects/spring-framework/pull/24785

Also there's one more place about which I'm not sure: This code

return DataBufferUtils.join(flux)
        .map(buffer -> {
            byte[] result = new byte[buffer.readableByteCount()];
            buffer.read(result);
            DataBufferUtils.release(buffer);
            return DigestUtils.md5DigestAsHex(result);
        });

could be rewritten as

return DataBufferUtils.join(flux)
        .map(buffer -> {
            try (InputStream inputStream = buffer.asInputStream(true)) {
                return DigestUtils.md5DigestAsHex(inputStream);
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        });

But I'm not sure whether it is worth the effort.

Comment From: rstoyanchev

Thanks for the pull request. I did some further refactoring along the same lines.