Rename ion-rs to ion-rs_old in preparation of upgrade

[jpschorr]

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

Conformance comparison report

	Base (deff7a8)	f23d693	+/-
% Passing	90.35%	90.35%	0.00%
✅ Passing	5731	5731	0
❌ Failing	612	612	0
🔶 Ignored	0	0	0
Total Tests	6343	6343	0

Number passing in both: 5731

Number failing in both: 612

Number passing in Base (deff7a8) but now fail: 0

Number failing in Base (deff7a8) but now pass: 0

[codecov]

Codecov Report

Attention: Patch coverage is 98.85057% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.48%. Comparing base (deff7a8) to head (3071a1e).
Report is 1 commits behind head on dev-ion-doc.

Files with missing lines	Patch %	Lines
extension/partiql-extension-ion/src/lib.rs	98.76%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           dev-ion-doc     #492      +/-   ##
===============================================
- Coverage        80.54%   80.48%   -0.07%     
===============================================
  Files               70       70              
  Lines            19040    18934     -106     
  Branches         19040    18934     -106     
===============================================
- Hits             15336    15239      -97     
- Misses            3257     3270      +13     
+ Partials           447      425      -22     

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