Pin rust version to 1.86.0 due to cairo_vm incompatibility (#3352)

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Closes #

## Introduced changes

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Interface of `is_multiple_of` changed in rust 1.87.0. This is a know
regression that won't be resolved, see
rust-lang/rust#140020

Due to that, `cairo_vm` no longer compiles on later rust version. Until
they upgrade and we upgrade our version of `cairo_vm`, then we have to
stay at rust 1.86.0

Issue to remove the pinned version
#3351

## Checklist

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- [x] Linked relevant issue
- [x] Updated relevant documentation
- [x] Added relevant tests
- [x] Performed self-review of the code
- [x] Added changes to `CHANGELOG.md`

Author: cptartur

rust-toolchain.toml

@@ -0,0 +1,3 @@
+[toolchain]
+channel = "1.86.0"
+components = ["rustfmt", "clippy"]