IsExternalInit isn't natively available in net standard 2.1

Currently IsExternalInit is if-defed to net standard 2.0 and below, but, apparently IsExternalInit isn't natively available in net standard 2.1.

Instead, IsExternalInit should be if-defed to NETSTANDARD2_1 || !NETSTANDARD2_1_OR_GREATER.

For this to work:

• the library will have to multitarget net standard 2.0 and 2.1.
• docs will have to be changed and the suggestion to conditionally include the library only in net standard 2.0 should be removed.
• the gitlab ci script needs to be changed to support the multitargeting.
• the docfx.json file needs a properties section added to build against the 2.0 target.