build: bump Symbolics compat

`maxiters` keyword to `fixpoint_sub` is required

Author: AayushSabharwal

Project.toml

@@ -131,7 +131,7 @@ SpecialFunctions = "0.7, 0.8, 0.9, 0.10, 1.0, 2"
 StaticArrays = "0.10, 0.11, 0.12, 1.0"
 SymbolicIndexingInterface = "0.3.31"
 SymbolicUtils = "3.7"
-Symbolics = "6.15.2"
+Symbolics = "6.15.4"
 URIs = "1"
 UnPack = "0.1, 1.0"
 Unitful = "1.1"