Deprecate `TF_ROOT` and introduce `GITLAB_TOFU_ROOT_DIR` as replacement

.gitlab/README.md.template

README.md

src/gitlab-tofu.sh

templates/apply.yml

templates/custom-command.yml

templates/destroy.yml

templates/fmt.yml

templates/graph.yml

templates/module-release.yml

templates/plan.yml

templates/test.yml

templates/validate.yml

tests/integration-tests/Defaults.gitlab-ci.yml

tests/integration-tests/Destroy.gitlab-ci.yml

...on-tests/FullPipelineTriggerInChildPipeline.gitlab-ci.yml

tests/integration-tests/JobTemplates.gitlab-ci.yml

tests/integration-tests/ModuleRelease.gitlab-ci.yml

tests/integration-tests/TestJob.gitlab-ci.yml

...s/ValidatePlanDestroyTriggerInChildPipeline.gitlab-ci.yml

tests/integration-tests/VarFile.gitlab-ci.yml