Sync upstream leanprover-community/iris-lean and bump to Lean 4.28.0

Merge upstream master (38 commits) bringing: Auth CMRA, HeapView, View
CMRA, internal fixpoints, coinduction, proof mode improvements (imod,
imodintro, ihave, ispecialize), IProp instances, GenMap refactor, and
Lean 4.28.0 toolchain.

Keep mathlib dependency (required by Bluebell) and bump to v4.28.0.
Fix DFrac name resolution in Permission.lean and Probability.lean
(DFrac_CMRA/op -> CMRA.op/DFrac.op) for compatibility with Lean 4.28.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: quangvdao

.github/workflows/build.yml

@@ -21,6 +21,6 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       # uses lean standard action with all default input values
       - uses: leanprover/lean-action@v1

.github/workflows/update-toolchain.yml

@@ -16,7 +16,7 @@ jobs:
       pull-requests: write # Grants permission to create or update pull requests
     steps:
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Update project
         uses: leanprover-community/lean-update@main
         with: