Add issue_branch_template attribute to gitlab_project resource

What does this MR do?

Adds the issue_branch_template attribute to the gitlab_project resource.

The attribute (template for branch names suggested when creating a branch from an issue, per the Projects API) is already exposed on the gitlab_project and gitlab_projects data sources but was missing from the resource. This wires the attribute through the schema, create, read, and update paths, mirroring the existing issues_template handling.

MR Checklist

• Attribute name matches the GitLab API 1:1 (issue_branch_template ↔️ IssueBranchTemplate on EditProjectOptions / Project in client-go).
• Acceptance test coverage added: extends TestAccGitlabProject_IssueMergeRequestTemplates with the new attribute, matching the existing inline config + assertion style for issues_template / merge_requests_template.
• make reviewable passes locally — build, gofmt, golangci-lint (0 issues), tfplugindocs generate, and unit tests are all green.
• Generated docs included (docs/resources/project.md).
• [N/A] examples/resources/gitlab_project/resource.tf not updated — the canonical example shows minimal usage and does not enumerate every attribute (sibling issues_template and merge_requests_template are likewise omitted).

Notes

• Acceptance tests requiring a live GitLab were not run locally (no TF_ACC=1 env). CI will exercise them.
• The existing TestAccGitlabProject_IssueMergeRequestTemplates uses bespoke check functions; the new assertion follows the same pattern for consistency with siblings in the same test function. Migrating that whole test to TestCheckResourceAttr + ImportStateVerify felt out of scope for an attribute-add MR but is a fair follow-up.
• Supersedes a previous draft (lu.cesarano fork) that could not run CI due to missing community-fork runner access.

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