fix(IDX): override mainnet_revisions.json (#3081)

The replica release `2d8611eb4efa8e69c4dd567546c1c353a545e0a6` was built
with Ubuntu-24.04 which means that it expects a newer version of libc to
be available in the environment where its binaries run. After deploying
this release we had to downgrade to Ubuntu 20.04 again. This means that
the binaries from the latest replica release don't work in our tests
since the latest libc is not available there. This effects the
`//rs/tests/consensus/backup:backup_manager_upgrade_test` for example.
We therefor temporarily override the mainnet revision to the commit just
before the Ubuntu 24.04 upgrade.

testnet/mainnet_revisions.json

@@ -1,6 +1,6 @@
 {
   "subnets": {
-    "tdb26-jop6k-aogll-7ltgs-eruif-6kk7m-qpktf-gdiqx-mxtrf-vb5e6-eqe": "2d8611eb4efa8e69c4dd567546c1c353a545e0a6",
+    "tdb26-jop6k-aogll-7ltgs-eruif-6kk7m-qpktf-gdiqx-mxtrf-vb5e6-eqe": "a88b490e10831ae42aea8d16c686fd46d9b1894f",
     "io67a-2jmkw-zup3h-snbwi-g6a5n-rm5dn-b6png-lvdpl-nqnto-yih6l-gqe": "d9fe2076f677a08734bed90c67b1c3f4056ed621"
   }
 }