deps: time zone updater workflow problems

[srl295]

See #47680 for example problem PR

• suggestion: move the script to tools/icu with other icu tools
• script should check the current tz version and not proceed if it's already >= the proposed tz version. This would have prevented deps: update timezone to 2023c #47680 from running.
• The PR should indicate the current ICU version, this would distinguish deps: update timezone to 2023c #47302 (ICU v72.1 and a legitimate update) from deps: update timezone to 2023c #47680 (ICU v73.1). It's visible from the icudt*.bz2 file which is updated.

Ref: #47288 for the original tool work.

[srl295]

@tibrewalricha fyi - thoughts?

[tibrewalricha]

The changes make sense to me. I would also recommend modifying maintaining-icu readme to ensure that future contributors know about this workflow.