/s/boost::beast::span/std::span

std::span is now available in c++ 20 builds, and should be a drop in
replacement for boost::span we were using previously.  This commit sed
replaces it, and changes reference to cbegin and cend to begin and end
respectively.

Tested:
Ran redfish-service-validator.  No new failures, and all nodes within
/redfish/v1/Registries that would be effected by this change respond
with 200 and the same content as previously.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Iace89473b7c20f32106eae9d872c16cfae5f17f6
diff --git a/redfish-core/lib/event_service.hpp b/redfish-core/lib/event_service.hpp
index d273aea..da9a165 100644
--- a/redfish-core/lib/event_service.hpp
+++ b/redfish-core/lib/event_service.hpp
@@ -416,13 +416,13 @@
                         // Check for Message ID in each of the selected Registry
                         for (const std::string& it : registryPrefix)
                         {
-                            const boost::beast::span<
+                            const std::span<
                                 const redfish::message_registries::MessageEntry>
                                 registry = redfish::message_registries::
                                     getRegistryFromPrefix(it);
 
                             if (std::any_of(
-                                    registry.cbegin(), registry.cend(),
+                                    registry.begin(), registry.end(),
                                     [&id](const redfish::message_registries::
                                               MessageEntry& messageEntry) {
                                         return !id.compare(messageEntry.first);