Use get_u16_str instead of snprintf in autoshift_timer_report

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Description

Including <string.h> for snprintf takes a considerable amount of firmware space. Using the newly added get_u16_str function lets us save some firmware size.

To assess the results, I measured the size regression using ./util/size_regression.sh on multiple keymaps that enable AUTO_SHIFT:

• atreus62/scheiklp delta: -802
• nacly/splitreus62/scheiklp delta: -1212
• dz60/LEdiodes delta: -1212
• handwired/dactyl_manuform/4x6/scheiklp delta: -1082
• unikeyboard/diverge3/iso_uk delta: -1150
• lily58/yshrsmz delta: +200
• massdrop/ctrl/xanimos delta: +0
• keyboardio/atreus/kkokdae delta: -766
• kprepublic/jj50/archetype delta: -1232
• crkbd/nimishgautam delta: +0
• xiudi/xd75/raoeus delta: -1210
• splitkb/kyria/jhelvy delta: -1214
• crkbd/rmeli delta: -1106
• xiudi/xd75/scheiklp delta: -1062
• handwired/dactyl/manuform/5x6/scheiklp delta: -1062

As we can see, about 1 kilobyte is saved in average. Lily58/yshrsmz is the only keymap whose size actually increases after applying this patch but I'm not sure why; perhaps snprintf is already used elsewhere in his keymap and this patch forces him to compile the get_u16_str function he previously didn't need.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

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The CI build failed due to an unrelated reason.

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Great, I can now actually flash auto shift onto my keyboard without removing other features 😄