update mmm-physics tag (ESCOMP#304)

Originator(s): peverwhee, kuanchihwang

Description (include issue title and the keyword ['closes', 'fixes',
'resolves'] and issue number):
Updates the .gitmodules tag for mmm-physics to match the submodule

List all namelist files that were added or changed: n/a

List all files eliminated and why: n/a

List all files added and what they do: n/a

List all existing files that have been modified, and describe the
changes:
(Helpful git command: `git diff --name-status
development...<your_branch_name>`)

M    .gitmodues
update mmm-physics tag to 20250616-MPASv8.3

List all automated tests that failed, as well as an explanation for why
they weren't fixed:

Is this an answer-changing PR? If so, is it a new physics package,
algorithm change, tuning change, etc?

If yes to the above question, describe how this code was validated with
the new/modified features:

Author: peverwhee

.gitmodules

@@ -1,7 +1,7 @@
 [submodule "mmm-physics"]
 	path = schemes/mmm/mmm_physics
 	url = https://github.com/NCAR/MMM-physics.git
-	fxtag = 20240626-MPASv8.2
+	fxtag = 20250616-MPASv8.3
 	fxrequired = AlwaysRequired
 	fxDONOTUSEurl = https://github.com/NCAR/MMM-physics.git
 [submodule "pumas"]