Even though the property is deprecated, it still shouldn't throw.

* Cherry pick changes on update_version.py #6643

* Update version to 3.10.0-rc-0

* Do not add rc0 to php packages.xml

* introduce GOOGLE_PROTOBUF_SUPPORT_SYSTEM_MEMORY

* allow building net45 target on unix systems too

* add Span<> support on net45 as well

Add comment explaining IExtensionMessage instability

 * Add Syntax enum and make file syntax checks check the enum
 * Made IsPacked a normal get method without ternary op
 * Moved IFieldAccessor.HasValue to IFieldPresenceAccessor
 * Change naming of GetIsExtensionsInitialized
 * Fixed stray text in summary text

Add ExtensionSet tests and add checks that non-message extension fields don't fail initialization checks

Adjust APIs for extensions to properly return default values for extensions
Fix issues with IsInitialized and proto2 field reflection

Fix generated group code missing end group tag case
Fix generated field classes searching for presence index for extensions