Clarify where to file Swift for TensorFlow issues

[dan-zheng]

The default issue template suggests that all Swift for TensorFlow issues should filed on bugs.swift.org:

We are using JIRA instead of GitHub issues to track our work in order to integrate more effectively with the rest of the Swift ecosystem.

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However, we've since changed in opinion:

• Swift compiler issues and crashes should filed on bugs.swift.org.
• Issues related to tensorflow/swift-apis (the TensorFlow module) should be filed as tensorflow/swift-apis GitHub issues.
• Issues related to tensorflow/swift-models should be filed as tensoflow/swift-models GitHub issues.
• Issues related to Swift for TensorFlow toolchains should be filed as tensorflow/swift GitHub issues, or on bugs.swift.org.

The default issue template should be updated to reflect this.