fix(deps): update rust crate cortex-m-rt to 0.7.3

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
cortex-m-rt	dependencies	patch	0.7.2 -> 0.7.3

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Release Notes

rust-embedded/cortex-m

v0.7.3

Compare Source

Fixed

• Fixed compilation for native targets on non-x86 host systems (#​336, #​337).

Added

• The Delay struct now offers direct delay_us() and delay_ms() methods
  without having to go through the embedded-hal traits (#​344).

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