fix update timestep print message

[JustinRayAngus]

This PR fixes a print issue introduced in PR #6840. When ApplyDtLimiters() is called, the updated time step is printed using:
amrex::Print() << Utils::TextMsg::Info("updating timestep to dt = " + std::to_string(dt[0]));

I find that the print looks like:
--- INFO : updating timestep to dt = 0.000000.

This PR instead uses

std::ostringstream oss;
oss << "updating timestep to DT = " << std::scientific << std::setprecision(6) << dt[0];
amrex::Print() << Utils::TextMsg::Info(oss.str());

Which looks like
--- INFO : updating timestep to DT = 1.000000e-12

It was also commented in PR #6840 that ApplyDtLimiters() is not applied at step 0, resulting in the time step set by the CFL for light waves. This PR also adds the call to ApplyDtLimiters() at step = 0.