BCGSTAB: return the iterate with the smallest residual

[bigfooted]

Proposed Changes

BCGSTAB does not reduce the residual monotonically. With a small iteration limit (e.g. LINEAR_SOLVER_ITER= 3) the last iterate can have a larger residual than the initial guess, and returning it gives a bad update. keeping the best iterate makes this converge for small iteration limits.

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.