fix(gke-cluster): Respect release channel when selecting GKE version by prefix

[kadupoornima]

This PR fixes an issue where the GKE cluster version selection logic was ignoring the specified release channel when a version_prefix was provided.

The Bug:
Previously, if min_master_version was not set, the module relied on the google_container_engine_versions data source and automatically selected latest_master_version. Because this data source queries versions without specifying a channel, it fetched the highest available patch version matching the given prefix across all channels. For example, if a user configured the REGULAR release channel, but the absolute latest matching version was currently only available in RAPID, Terraform would attempt to deploy a version that had not yet rolled out to REGULAR, which could result in an API error during cluster creation.

The Fix:
This PR updates the internal logic for setting the GKE version to factor in the release channel. It replaces the hardcoded latest_master_version fallback with the following conditional logic:

• If a specific release_channel is configured (i.e., not "UNSPECIFIED"), it now uses the data source's release_channel_latest_version attribute to retrieve the latest version specific to that channel.
• It only falls back to the generic latest_master_version if the release channel is left as "UNSPECIFIED".

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a bug in how GKE cluster versions are resolved within the scheduler module. By introducing conditional logic for release channels, the module now accurately fetches the appropriate version based on the user's configuration. Additionally, minor formatting improvements were applied to the module's documentation.

Highlights

• GKE Version Resolution: Updated the GKE master version logic to correctly account for release channels, ensuring the latest version is selected based on the specified channel rather than defaulting to the latest master version.
• Documentation Cleanup: Standardized Markdown table formatting in the README for improved readability and consistency.

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[gemini-code-assist]

Code Review

This pull request updates the GKE cluster module to dynamically determine the master version based on the configured release channel when it is not set to 'UNSPECIFIED'. It also formats markdown tables in the module's README. A review comment correctly identifies a potential Terraform evaluation error when performing a direct map lookup on release_channel_latest_version if the key is missing, and suggests using the lookup() function with a fallback to prevent failures.