Fix.template vars section

[wxtim]

Does not close a separate issue;

Issue

Reported by @james-bruten-mo:

Rose stem assumes that the name of the templating section is [jinja2:suite.rc]. At Rose 2 [template variables] is preferred, though the older form is kept for back compatibility. If the new form is used in the config file rose stems use of the older form will cause one of cylc-roses checks on the config to fail.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• CHANGES.md entry included if this is a change that can affect users
• Doc changes are not needed.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[MetRonnie]

Looks ok to me but I don't know how to test

[oliver-sanders]

Milestone doesn't match branch, which way are we going with this one?