Make propt::solver_text const

[tautschnig]

There is no point in marking the string returned by value const, it's really about marking the method itself const.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Base: 78.00% // Head: 78.00% // No change to project coverage 👍

Coverage data is based on head (400c03e) compared to base (abf93f5).
Patch coverage: 57.14% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7260   +/-   ##
========================================
  Coverage    78.00%   78.00%           
========================================
  Files         1621     1621           
  Lines       187273   187273           
========================================
  Hits        146089   146089           
  Misses       41184    41184           

Impacted Files	Coverage Δ
src/solvers/prop/prop.h	83.78% <ø> (ø)
src/solvers/sat/cnf_clause_list.h	80.00% <0.00%> (ø)
src/solvers/sat/external_sat.h	0.00% <ø> (ø)
src/solvers/sat/satcheck_minisat2.h	75.00% <ø> (ø)
unit/solvers/bdd/miniBDD/miniBDD.cpp	90.06% <0.00%> (ø)
src/solvers/sat/dimacs_cnf.h	27.27% <50.00%> (ø)
src/solvers/sat/external_sat.cpp	88.09% <100.00%> (ø)
src/solvers/sat/satcheck_minisat2.cpp	63.15% <100.00%> (ø)

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