Replace nonstd::optional by C++17 std::optional

[tautschnig]

Removes third-party code that is no longer necessary now that we use C++ 17 as build standard. Future uses of optionalt should instead use std::optional directly.

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• 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

All modified and coverable lines are covered by tests ✅

Comparison is base (ee33bc8) 79.09% compared to head (68633ab) 79.06%.

❗ Current head 68633ab differs from pull request most recent head 633010a. Consider uploading reports for the commit 633010a to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8034      +/-   ##
===========================================
- Coverage    79.09%   79.06%   -0.03%     
===========================================
  Files         1699     1697       -2     
  Lines       196503   196364     -139     
===========================================
- Hits        155419   155261     -158     
- Misses       41084    41103      +19     

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[esteffin]

Looks good except the failure on ubuntu 22.04 and gcc 13 and marking the legacy optionalt as deprecated

[feliperodri]

LGTM.