vale-styles: Upgrade all checks to "error" level

- I added these all the way back in December 2019 in #6826, but only
  today realised that things have been failing unnoticeably because
  "warning" level exits with status code 0, so doesn't fail CI.
- Upgrading to "error" level will enable us to see the style errors in
  our docs and fix them faster.
- My editor also auto-formatted the YAML to have double quotes instead
  of single.

docs/vale-styles/Homebrew/Abbreviations.yml

@@ -2,7 +2,7 @@
 extends: substitution
 message: Use '%s'
 ignorecase: false
-link: 'https://github.com/Homebrew/brew/blob/HEAD/docs/Prose-Style-Guidelines.md#style-and-usage'
+link: "https://github.com/Homebrew/brew/blob/HEAD/docs/Prose-Style-Guidelines.md#style-and-usage"
 level: error
 nonword: true
 swap:

docs/vale-styles/Homebrew/OxfordComma.yml

 ---
 extends: existence
-message: 'No Oxford commas!'
-link: 'https://github.com/Homebrew/brew/blob/HEAD/docs/Prose-Style-Guidelines.md#typographical-conventions'
+message: "No Oxford commas!"
+link: "https://github.com/Homebrew/brew/blob/HEAD/docs/Prose-Style-Guidelines.md#typographical-conventions"
 scope: sentence
-level: warning
+level: error
 tokens:
   - '(?:[^,]+,){1,}\s\w+,\sand'

docs/vale-styles/Homebrew/Pronouns.yml

 ---
 extends: existence
 message: Avoid gender-specific language when not necessary.
-link: 'https://github.com/Homebrew/brew/blob/HEAD/docs/Prose-Style-Guidelines.md#personal-pronouns'
-level: warning
+link: "https://github.com/Homebrew/brew/blob/HEAD/docs/Prose-Style-Guidelines.md#personal-pronouns"
+level: error
 ignorecase: true
 tokens:
   - him

docs/vale-styles/Homebrew/Spacing.yml

 ---
 extends: existence
 message: "'%s' should have one space."
-link: 'https://github.com/Homebrew/brew/blob/HEAD/docs/Prose-Style-Guidelines.md#typographical-conventions'
+link: "https://github.com/Homebrew/brew/blob/HEAD/docs/Prose-Style-Guidelines.md#typographical-conventions"
 level: error
 nonword: true
 tokens:
-  - '[a-z][.?!][A-Z]'
-  - '[.?!] {2,}[A-Z]'
+  - "[a-z][.?!][A-Z]"
+  - "[.?!] {2,}[A-Z]"

docs/vale-styles/Homebrew/Terms.yml

 ---
 extends: substitution
 message: Use '%s' instead of '%s'.
-link: 'https://github.com/Homebrew/brew/blob/HEAD/docs/Prose-Style-Guidelines.md#terminology-words-and-word-styling'
+link: "https://github.com/Homebrew/brew/blob/HEAD/docs/Prose-Style-Guidelines.md#terminology-words-and-word-styling"
 level: error
 scope: $paragraph
 swap:

docs/vale-styles/Homebrew/Titles.yml

 extends: capitalization
 message: "'%s' should be in title case"
-level: warning
+level: error
 scope: heading.h1
 match: $title
 style: AP

docs/vale-styles/Homebrew/Trademarks.yml

 ---
 extends: existence
 message: 'No "TM", ™, SM, ©, ®, or other explicit indicators of rights ownership or trademarks'
-link: 'https://github.com/Homebrew/brew/blob/HEAD/docs/Prose-Style-Guidelines.md#typographical-conventions'
+link: "https://github.com/Homebrew/brew/blob/HEAD/docs/Prose-Style-Guidelines.md#typographical-conventions"
 level: error
 nonword: true
 tokens: