chore(gae): migrate region tags from docs/appengine/datastore/indexes/index.yaml

[OremGLG]

Description

Migrate region tags from docs/appengine/datastore/indexes/index.yaml in order to align to pattern "product_regiontag_typeoffile_language"

Fixes
b/394594341
b/394594345

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Checklist

• I have followed Contributing Guidelines from CONTRIBUTING.MD
• Tests pass: go test -v ./.. (see Testing)
• Code formatted: gofmt (see Formatting)
• Vetting pass: go vet (see Formatting)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• docs/appengine/datastore/indexes/index.yaml:15, tag gae_datastore_exploding_index_2_yaml_go
• docs/appengine/datastore/indexes/index.yaml:26, tag gae_datastore_exploding_index_4_yaml_go

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