fix(Dashboard): Backward compatible shared_label_colors field

[geido]

SUMMARY

A follow-up PR for #30646. This PR ensure that a dashboard that used the shared_label_colors field as a dict is compatible with the shared_label_colors being a list as per the latest changes and can be re-imported / copied without errors.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N.A.

TESTING INSTRUCTIONS

1. Export a Dashboard before changes in fix(Dashboard): Retain colors when color scheme not set #30646
2. Re-import the Dashboard after changes in fix(Dashboard): Retain colors when color scheme not set #30646
3. Copy the dashboard and all of its charts utilizing "Save as"
4. Copying the dashboard should work without errors

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API