fix(DateTimePickerv2): update placeholder text to show XM instead of 'a' or 'A'

Closes #3762

Summary

TimePickerDropDown follows the carbon guideline to show placeholder text like hh:mm XM if it is 12hours format. To be consistent, we update DateTimePickerV2 single select placeholder text to show XM instead of a or A.
It should look like YYYY-DD-MM hh:mm XM

Change List (commits, features, bugs, etc)

Update in parseValue() in dateTimePickerUtils.js, specifically to single select.

Acceptance Test (how to verify the PR)

Regression Test (how to make sure this PR doesn't break old functionality)

Things to look for during review

[ ] Make sure all references to iot or bx class prefix is using the prefix variable
[ ] (React) All major areas have a data-testid attribute. New test ids should have test written to ensure they are not changed or removed.
[ ] UI should be checked in RTL mode to ensure the proper handling of layout and text.
[ ] All strings should be translatable.
[ ] The code should pass a11y scans (The storybook a11y knob should show no violations). New components should have a11y test written.
[ ] Unit test should be written and should have a coverage of 90% or higher in all areas.
[ ] All components should be passing visual regression test. For new styles or components either a visual regression test should be written for all permutations or the base image updated.
[ ] Changes or new components should either write new or update existing documentation.
[ ] PR should link and close out an existing issue

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carbon-design-system / carbon-addons-iot-react