[CI] Grenade job changes for 2024.2 release cycle

We're in a non-SLURP release cycle; this means that we'd test upgrading to
the release produced by this cycle is supported only from stable/2024.1 (Caracal).

So set "grenade_from_branch" to stable/2024.1 and run only the "skip-level"
grenade job. When we get into the 2025.1 cycle, we can run the regular
grenade job as well.

Change-Id: If17ddd1bc06810d94db43dec3bb877d5045621ad
Signed-off-by: Goutham Pacha Ravi <gouthampravi@gmail.com>

zuul.d/grenade-jobs.yaml

@@ -91,4 +91,4 @@
     vars:
       # Move this forward when a new release cycle start. grenade_from_branch
       # should be always N-2 from master.
-      grenade_from_branch: stable/2023.1
+      grenade_from_branch: stable/2024.1

zuul.d/project.yaml

@@ -23,7 +23,9 @@
             voting: false
         - manila-tempest-plugin-lvm-fips:
             voting: false
-        - manila-grenade
+        # (TODO) gouthamr: unskip this grenade job when trunk is 2025.1 since
+        # this is different from the job below only during SLURP releases
+        # - manila-grenade
         - manila-grenade-skip-level-always
         - manila-rally-no-ss:
             voting: false