Update Jinja2 to 3.0.*

[MetRonnie]

These changes close #4702

Update Jinja2 from 2.11 to 3.0

Requirements 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.
• Does not need tests (tested manually against many existing Cylc 7 source suites).
• Appropriate change log entry included.
• (master branch) I have opened a documentation PR: Add note on Jinja2 zero-padded integers cylc-doc#467

[MetRonnie]

Do we want to include a complex jinja2 workflow example that we diff against a reference file, as a functional test?

[oliver-sanders]

Do we want to include a complex jinja2 workflow example that we diff against a reference file, as a functional test?

I don't think so and we wouldn't be able to capture all uses and patterns. Hopefully future upgrades won't be so complicated. The timing of this one with Cylc 8 and the need for 7/8 interoperability has made this tricky.

[oliver-sanders]

Note to self: remember to remove this in the conda-feedstock.

[hjoliver]

conda-forge/cylc-flow-feedstock#42

[oliver-sanders]

LGTM but haven't manually tested.

[MetRonnie]

functional tests / macos failed due to Codecov upload failure only