echidna: replace multi-abi with allContracts in configs

Echidna renamed its `multi-abi` option to `allContracts` in commit
ff312a6 ("Rename multi-abi to allContracts"). This adds the new
option to the test configuration to keep CI passing, while
maintaining references in text to the currently available option to
not confuse users. A second PR should adjust the text to mention
the new option once Echidna 2.1 is released.

Author: elopez

program-analysis/echidna/example/multiabi.yaml

@@ -1,3 +1,6 @@
 testMode: assertion
 testLimit: 50000
-multi-abi: true
+# echidna < 2.1
+multi-abi: true
+# echidna >= 2.1
+allContracts: true