Add a maximum value for trait MultiplierUpdate

[kianenigma]

... and enforce it in all implementations, including the one being added in #12222

Outcome should be:

/// Something that can convert the current multiplier to the next one.
pub trait MultiplierUpdate: Convert<Multiplier, Multiplier> {
	/// Minimum multiplier. Any outcome of the `convert` function should be more than this.
	fn min() -> Multiplier;
	/// Maximum multiplier. Any outcome of the `convert` function should be less than this.
	fn max() -> Multiplier;
	/// Target block saturation level. The `convert` function could possibly update itself based on this target.
	fn target() -> Perquintill;
	/// Variability factor. The `convert function` could possible tune its speed of update based on this.
	fn variability() -> Multiplier;
}

This would help implementations ensure that even if the fees should increase forever, they will not go above a certain threshold.

This remains backwards compatible by setting the maximum to Bounded::max_value().