refactor[integrations/utils.ts]: change let to const for those references doesn't change

[samuel871211]

Some variables were declared with the let keyword but never reassigned. For the reasons below, it would be a better practice to change to const.

1. Prevents accidental reassignment
2. Signals intent clearly to other developers
3. Improves code readability
4. Enables potential compiler optimizations
5. Simplifies debugging
6. Promotes immutability
7. Reduces bugs from mutable state

[RobinMalfait]

More info: #17567 (comment)