Feat/dashboard style variants

[rolfsmeds]

Description

• Elevated and Flat style variants
• Shaded background variant for board itself

Fixes #8961

Continuation of vaadin/web-components#8962

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/contributing/pr
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.
• I have not completed some of the steps above and my pull request can be closed immediately.

Additional for Feature type of change

• Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[vaadin-bot]

Hi @rolfsmeds and @ugur-vaadin, when i performed cherry-pick to this commit to 24.8, i have encountered the following issue. Can you take a look and pick it manually?
Error Message:
Error: Command failed: git cherry-pick ad6eb0d
error: could not apply ad6eb0d... Feat/dashboard style variants (#7317)
hint: After resolving the conflicts, mark them with
hint: "git add/rm ", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".