chore(vscode-settings): remove deprecated property; disable postcss validate

[cdransf]

Description

• Removes deprecated aslant.packageManager setting
• Adds postcss.validate: false as postcss.validate conflicts with our stylelint and posts

How and where has this been tested?

Verified locally. Removing the following:

"at-rule-no-unknown": [
  true,
  {
    ignoreAtRules: ["extend", "container", "each", "include", "mixin"],
  },
],

Raises a lint violation against at-rule-no-unknown, establishing that ignoreAtRules is being applied (as no exceptions are now being defined in the rule definition).

Restoring the rule then returns our unknownAtRules violation which is attributed to postcss.

Adding postcss.validate: false to the workspace settings resolves the issue. Additionally — removing "extend" from ignoreAtRules and restarting the stylelint server in VS Code causes stylelint to surface a violation — we can then rely on stylelint to provide coverage for syntax violations like this.

To-do list

• [x] I have read the contribution guidelines.
• [ ] I have updated relevant storybook stories and templates.
• [ ] I have tested these changes in Windows High Contrast mode.
• [ ] If my change impacts other components, I have tested to make sure they don't break.
• [ ] If my change impacts documentation, I have updated the documentation accordingly.
• [x] ✨ This pull request is ready to merge. ✨

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Latest commit: 58af8d44c67df7d7b789a776397096bdd773642b

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