fix(crypto): Rename the device keys property for Olm messages

[poljar]

Commit d41af39 implemented MSC4147, which puts the device keys into a Olm message. It failed to adhere to the unstable prefix proposed in the MSC:

Until this MSC is accepted, the new property should be named
org.matrix.msc4147.device_keys.

This patch fixes this.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.16%. Comparing base (d41af39) to head (ceec2a3).

Files	Patch %	Lines
crates/matrix-sdk-crypto/src/olm/session.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3618      +/-   ##
==========================================
- Coverage   84.19%   84.16%   -0.04%     
==========================================
  Files         255      255              
  Lines       26416    26416              
==========================================
- Hits        22241    22232       -9     
- Misses       4175     4184       +9     

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