Improve issue weight system notes

What does this MR do and why?

Improve issue weight system notes #409673 (closed)

Show the old weight as well as the new one (and differentiate between setting weight, removing weight and changing weight).

This goes hand in hand with a handful of other merge requests to provide better visibility/accountability for billing/audit purposes.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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Before	After

How to set up and validate locally

Note: You'll need an EE license to unlock issue weights in GDK

1. Find an issue
2. Add a weight
3. Change the weight
4. Remove the weight
5. Check the notes look like:

This will be for all old issues (depending on whether we end up backfilling). We aren't populating the new field yet (a new MR will be up shortly to add this). To test how they "will look":

1. Find an issue
2. Add a weight 10
3. Change the weight to 20
4. From the rails console, run:

   e = ResourceWeightEvent.last
   e.previous_weight = 10
   e.save

5. Remove the weight
6. From the rails console, run:

   e = ResourceWeightEvent.last
   e.previous_weight = 20
   e.save

7. Check the notes look like: