Modernise 'cylc dump -t' print format.

[hjoliver]

This has been annoying me forever.

Master 🤮:

$ cylc dump -t bug --flows
foo, 100, running, not-held, not-queued, not-runahead, [1]
foo, 101, waiting, not-held, not-queued, runahead, [1]
foo, 96, running, not-held, not-queued, not-runahead, [1]
foo, 97, running, not-held, not-queued, not-runahead, [1]
foo, 98, running, not-held, not-queued, not-runahead, [1]
foo, 99, running, not-held, not-queued, not-runahead, [1]

This branch:

$ cylc dump -t bug --flows
91/foo:running flows=[1]
92/foo:running flows=[1]
93/foo:running flows=[1]
94/foo:running flows=[1]
95/foo:running flows=[1]
96/foo:waiting (runahead) flows=[1]

QUESTION: is it OK to make the new format the default?
(As I have done, with -l/--legacy to generate the old format)

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).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.