Capitalize immutable variables

[PaulRBerg]

Immutable variables are constant at runtime, so they should be named like constants in UPPERCASE_SNAKE_CASE.

Solhint has a rule for this, which is currently disabled in our Solhint config file. Activating it finds several occurrences:

https://app.warp.dev/block/782axtYGfdaPdIxf8mHIAY

We should refactor and then activate the rule (by removing this line from the Solhint config).