chore: update go to v1.25

[fabiomatavelli-f3]

Summary

Update Go to v1.25, modernise CI workflows, and improve the provider reliability.

Ref.: form3tech/project-ctp#9

Changes

Go & Dependencies

• Upgrade Go from 1.18 to 1.25 in go.mod
• Promote golang.org/x/oauth2 to a direct dependency
• Vendor dependencies committed to the repository (removed vendor/ from .gitignore)

CI/CD Workflows

• ci.yaml: Simplified and modernised the CI pipeline
  • Replaced pinned-but-outdated GitHub Actions with newer versions (actions/checkout@v4, actions/setup-go@v5)
  • Added proper permissions (id-token, contents, pull-requests)
  • Added concurrency control to cancel in-progress runs for the same branch
  • Removed the publish job (moved to a dedicated release workflow)
  • Environment variables (TF_ACC, COMMIT_MESSAGE_PREFIX, GITHUB_TOKEN, etc.) are now set at the workflow level
• release.yaml (new): Added a dedicated release workflow that triggers on pushes to master, using the shared form3tech/tfe-providers-gh-workflows reusable workflow

Bug Fixes

• Branch support in commits: Pass the Branch field to commit.CreateCommit in both create/update and delete operations, ensuring commits target the correct branch instead of defaulting
• Archived repository handling: When deleting a file resource, check if the target repository is archived first. If it is, gracefully skip the delete and remove the resource from state instead of failing

Tests

• Self-contained tests: Tests now run against the same repository (form3tech-oss/terraform-provider-githubfile) on a dynamically created branch (acc-test-<random>), eliminating the dependency on the external terraform-provider-githubfile-test repository
• Automatic cleanup: A TestMain function ensures the test branch is deleted after all tests complete
• Unit tests for delete behaviour: Added TestResourceFileDelete_ArchivedRepo and TestResourceFileDelete_NonArchivedRepo_FileNotFound using a mock HTTP server to validate the archived-repo and file-not-found edge cases

Misc

• Quoted variable references in Makefile test target for robustness
• Removed GO111MODULE=on env from goreleaser.yaml (unnecessary with Go 1.25)