Add SECURITY_MANAGER to allowedBaseAccessLevels in gitlab_member_role

What does this MR do?

Adds SECURITY_MANAGER as a valid value for base_access_level in the gitlab_member_role resource. Previously, the provider rejected SECURITY_MANAGER (access level 25) even though it is a supported value in the GitLab Member Roles API.

Changes

In resource_gitlab_member_role.go, SECURITY_MANAGER was added to the allowedBaseAccessLevels slice. In resource_gitlab_member_role_test.go, a new acceptance test TestAccGitlabMemberRole_SecurityManager was added covering create, update, import, and destroy-before-create replacement behavior. In docs/resources/member_role.md, SECURITY_MANAGER was added to the list of valid base_access_level values in the documentation.

Related issue

Closes #6846 (closed)