Sync gigasecond

[mk-mxp]

• Run bin/configlet sync -u -e gigasecond --yes --docs --filepaths --metadata --tests include (updates the Markdown files and maybe tests.toml)
• Drop strict types comments from test file and example code (these are useless)
• Combine testFrom() and testFromReturnType() into one test method to have one method per test case
• Inline dateSetup($date) into test method, so students have all code in one place
• Copy test method for each test case from problem-spec (see Discussion: Use of data providers in test classes #755)
• Add and sync test meta data to tests (uuid / #TestDox in DocBlocks)
• Add exercise to bin/auto-sync.txt to keep it in sync (keep the file alphabetically sorted)
• Decide on adding / adjusting / ordering test cases to match current problem specs
  • gigasecond: missing 1 test case

Do not redesign the student's interface or add test cases that would invalidate existing community solutions. These are extra tasks, which should be discussed in advance.