[Pre537] Change github_advisory_sync.rb script to conform to project style guidelines

[jasnow]

Change github_advisory_sync.rb script to conform to project style guidelines.

Such as:
a. Fix indentation for related:/url:, patched_versions: and unafffected_versions fields.

  * "patched_version:" field needs to be moved over 4 spaces from left margin.
  * "related"/"url:" lines need to be moved over 2 spaces more (for total of 4 spaces from left margin).

b. Use of double quotes over single quotes

• Exception: Can use single quotes when the text contains a period (".") or colon (":").

c. Fix description: multi-line format syntax.
d. Remove extra "url:" from related:/url: values.
e. Line wrap description: field to 80 columns.

• Expect more to come

[postmodern]

Ruby's YAML library will prefer double-quotes to single-quotes. Appears that single-quotes are only used when the String contains a : character. I think it's acceptable to allow single-quotes in that edge-case.