chore: delete old strategy variants; use new variants in default strategy

[thomasheartman]

Instead of the old variants, let's use the new variants for the default strategy.

From manual testing, they appear to work almost the exact same. My guess is that the old version was just forgotten after #5850.

The permissions that we used to require are the same permissions as the ones required to save the form, so there's no need to but those permissions on the "add variants" button.

In all the following screenies, the new strategy variants version is shown on the left, the old version on the right:

No variants:

Open editing:

Custom weighting:

Without names:

with names:

saved:

Claude's summary:

Here are the key differences between the two components:

1. Initialization pattern

StrategyVariants initializes variants in a useEffect with empty dependency array, setting state after mount. NewStrategyVariants initializes variants directly in the useState call, avoiding the extra render cycle.

2. Cleanup effect

NewStrategyVariants has an additional cleanup useEffect (return function) that filters out variants with empty names when the component unmounts. StrategyVariants has no such cleanup.

3. Weight validation

NewStrategyVariants computes a variantWeightsError boolean (checks if weights don't sum to 1000) and passes it to both VariantForm (as weightsError) and VariantsSplitPreview (as weightsError). StrategyVariants does no weight validation.

4. Permission prop

StrategyVariants accepts a configurable permission prop (defaulting to UPDATE_FEATURE_ENVIRONMENT_VARIANTS). NewStrategyVariants hardcodes UPDATE_FEATURE_ENVIRONMENT_VARIANTS directly.

5. UI differences

• NewStrategyVariants shows an Alert info banner at the top explaining what variants do.
• The help tooltip text is longer and more descriptive in NewStrategyVariants.
• The help icon uses a styled Box wrapper (StyledHelpIconBox) instead of being inline in the h3.
• The "Add variant" button has a startIcon={<Add />} icon in NewStrategyVariants.
• StrategyVariants renders the heading as an <h3> via Typography variant='h3'; NewStrategyVariants uses a plain Typography (no heading variant).
• The StrategyVariantsUpgradeAlert is conditionally rendered only when variants exist in NewStrategyVariants, but always shown in StrategyVariants.
• NewStrategyVariants uses a plain <a> tag for the docs link; StrategyVariants uses MUI's <Link>.

In summary: NewStrategyVariants is a revised version with better initialization (no unnecessary re-render), weight validation, cleanup on unmount, and a refreshed UI. StrategyVariants is the older version with more flexibility (configurable permission) but fewer guardrails.

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[thomasheartman]

This adds the parameterized permission back in, defaulting to what was the hardcoded value before. It's likely that the implementation will change a bit when merging it with milestone strategy variants, so this is likely to be a stopgap.

[thomasheartman]

These permissions are already required to save the form itself. It doesn't really make any sense to gate this one form field behind permissions you already need.