CONTRACTS: do not rely on replace_symbol for bound variables [blocks: #6827]

[tautschnig]

replace_symbolt will cease to support bound variables, which presently
is not well documented behaviour. See #6827 for upcoming changes.

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• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
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[codecov]

Codecov Report

Merging #6828 (ed4b77f) into develop (55f3861) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6828      +/-   ##
===========================================
- Coverage    77.01%   77.01%   -0.01%     
===========================================
  Files         1594     1594              
  Lines       184417   184415       -2     
===========================================
- Hits        142021   142019       -2     
  Misses       42396    42396              

Impacted Files	Coverage Δ
src/goto-instrument/contracts/contracts.h	100.00% <ø> (ø)
src/goto-instrument/contracts/contracts.cpp	94.48% <100.00%> (-0.01%)	⬇️
src/solvers/flattening/boolbv.cpp	80.33% <100.00%> (-0.06%)	⬇️

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[feliperodri]

LGTM

[kroening]

The use of the .instantiate() API is certainly an improvement. It's not clear to me why a fresh symbol is needed, though.