chore(firestore): update region tags to use product prefix
chore(firestore): update region tags to use product prefix

Note: these are not used, according to my tracking sheet, so let's see if SnippetBot agrees. :) 

Context: go/firestore-region-tags-normalization-2020q4
GoogleCloudPlatform/dotnet-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,GoogleCloudPlatform/dotnet-docs-samples