Merge branch 'atmosphere/update_mmm_phys_tag' into hotfix-v8.3.1 (PR #1337)

This merge updates the MMM-physics external tag to 20250616-MPASv8.3 in the
src/core_atmosphere/Externals.cfg file to address an issue with .F90 files not
being re-compiled to .o files if a .F90 file was modified. With the updated tag,
compiling the atmosphere core, then making changes to any of the .F90 files in
src/core_atmosphere/physics/physics_mmm/, then running make again (without first
cleaning) leads to the modified .F90 files being correctly re-compiled.

* atmosphere/update_mmm_phys_tag:
  Update MMM-physics tag in Externals.cfg to fix .F90 re-compilation issue

Author: mgduda

src/core_atmosphere/Externals.cfg

@@ -2,7 +2,7 @@
 local_path = ./physics_mmm
 protocol = git
 repo_url = https://github.com/NCAR/MMM-physics.git
-tag = 20240626-MPASv8.2
+tag = 20250616-MPASv8.3
 required = True
 
 [GSL_UGWP]