Avoid hardcoding boost::ecuyer1988

[WardBrian]

Submission Checklist

• [x] Run unit tests
• Documentation
  • [ ] If a user-facing facing change was made, the documentation PR is here:
  • [x] OR, no user-facing changes were made

Release notes

Follow on to https://github.com/stan-dev/stan/pull/3263. The generated C++ no longer assumes that the RNG used in Stan is specifically the boost::ecuyer1988 generator, but uses a type alias defined in the Stan library.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8f2dbe7) 89.88% compared to head (6ac9fde) 89.88%. Report is 6 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1395 +/- ## ======================================= Coverage 89.88% 89.88% ======================================= Files 63 63 Lines 10505 10505 ======================================= Hits 9442 9442 Misses 1063 1063 ``` | [Files](https://app.codecov.io/gh/stan-dev/stanc3/pull/1395?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=stan-dev) | Coverage Δ | | |---|---|---| | [src/stan\_math\_backend/Lower\_functions.ml](https://app.codecov.io/gh/stan-dev/stanc3/pull/1395?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=stan-dev#diff-c3JjL3N0YW5fbWF0aF9iYWNrZW5kL0xvd2VyX2Z1bmN0aW9ucy5tbA==) | `98.64% <100.00%> (ø)` | | | [src/stan\_math\_backend/Lower\_program.ml](https://app.codecov.io/gh/stan-dev/stanc3/pull/1395?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=stan-dev#diff-c3JjL3N0YW5fbWF0aF9iYWNrZW5kL0xvd2VyX3Byb2dyYW0ubWw=) | `99.18% <100.00%> (ø)` | |