Convert old clock triggers to wall_clock xtriggers.

[hjoliver]

Close #5285

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.
• Tests are included (or explain why tests are not needed).
• CHANGES.md entry included if this is a change that can affect users
• Cylc-Doc pull request opened Update old-style clock-trigger warning. cylc-doc#575
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

Covered by existing tests:

$ git grep 'clock-trigger' tests/*functional
tests/flakyfunctional/events/40-stall-despite-clock-trig/flow.cylc:        clock-trigger = t1(P0D)
tests/flakyfunctional/special/04-clock-triggered/flow.cylc:        clock-trigger = clock{% if OFFSET is defined %}({{OFFSET}}){% endif %}
tests/functional/clock-trigger-inexact/00-big-offset/flow.cylc:# Test clock-trigger offset involving inexact intervals (months and years),
tests/functional/clock-trigger-inexact/00-big-offset/flow.cylc:        clock-trigger = baz({{OFFSET}})  # old-style clock-triggered task
tests/functional/cylc-config/00-simple/section1.stdout:    clock-trigger =
tests/functional/cylc-show/clock-triggered-non-utc-mode/flow.cylc:        clock-trigger = foo(PT5M)
tests/functional/runahead/release-update/flow.cylc:        clock-trigger = foo(P0Y)
tests/functional/special/07-clock-triggered-360.t:grep_ok "WorkflowConfigError: clock-trigger tasks require \[scheduling\]cycling mode=" \
tests/functional/special/clock-360/flow.cylc:        clock-trigger = clock(0)
tests/functional/validate/35-pass-special-tasks-non-word-names.t:        clock-trigger = t-1, t+1, t%1, t@1
tests/functional/validate/37-special-implicit-task.t:        clock-trigger = foo(PT0M)

[hjoliver]

Two functional tests deleted because they only tested cylc show output for old-style clock-triggers.

[oliver-sanders]

That was quick!

[oliver-sanders]

Nice, tested as working.

[oliver-sanders]

We might want to remove the ClockTriggerTasks section of the docs along with this.

[MetRonnie]

LGTM (just codecov falling over as is its wont)