[MAINT] migrate github_branch_default to context-aware CRUD functions and tflog

[ElenaZvereva]

Part of #2996 and #3070

Before the change?

• Legacy CRUD functions (Create, Read, Update, Delete) were used in resource_github_branch_default.go.
• Go's standard log package was used for logging.

After the change?

• Migrated to context-aware CRUD functions (CreateContext, ReadContext, UpdateContext, DeleteContext).
• Replaced log usage with HashiCorp's structured logging package tflog.

Pull request checklist

• Schema migrations have been created if needed (example)
• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[github-actions]

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[deiga]

Would you be available to rebase this (possibly multiple times) during the next week? We're going to release 6.13.0 and will try to land some of these open PRs

[Copilot]

Pull request overview

Migrates the github_branch_default Terraform resource to Terraform Plugin SDK v2 context-aware CRUD handlers and switches logging to tflog, aligning this resource with the provider’s ongoing context + structured logging modernization efforts.

Changes:

• Switched resource handlers from legacy Create/Read/Update/Delete to CreateContext/ReadContext/UpdateContext/DeleteContext.
• Updated CRUD implementations to accept context.Context and return diag.Diagnostics (using diag.FromErr for error propagation).
• Replaced log.Printf with structured tflog.Info logging for state-removal on 404s.

[ElenaZvereva]

Yes, I'll do this