Ported changes from Umbraco 13 around not throwing exceptions with identity IDs that can't be parsed to an integer or GUID

[AndyButland]

This PR ports over the updates made in:

• Avoid an exception on sign out when the principal is populated from an incomplete external login #18078
• Avoid exception when attempting to find member by Id when Id is not an expected Guid or integer, as can be the case with external member providers #18320

The were resolving reports of issues with external login providers where we would have an identity id that wasn't an integer or GUID. These would fail in conversion in the attempts to find a local user matching those IDs.

• Umbraco issue when using Okta #14713
• Auto linking and manual linking Google account with external provider fails #18002

Given the rework in this area these didn't merge up from 13, so I'm re-implementing them here.

I've also reverted back the changes to UmbracoUserStore.UserIdToInt that came through in the merge, but aren't necessary as the method in 15 is obsolete.

To Test:

• I'd suggest that given this is really an addendum to the merge from 13, visual inspection will suffice.