Update outdated examples and versions in README

[ekalinin]

README contains several outdated examples and versions.

Outdated items

• Usage and "Auto insert and update TOC" examples show TOC output in an old
  format (2 and 8 space indents). The current default is --indent 3: top-level
  entries have no indent, nested ones use 3, 6, ... spaces (see tests/tests.bats).
• README's own TOC between <!--ts--> and <!--te--> was generated by an older
  version: its indentation differs from the current output.
• "TOC generation with Github Actions" uses actions/checkout@v2 and
  stefanzweifel/git-auto-commit-action@v4. The latest releases are v7.0.1 and
  v7.2.0.
• Docker / Public pulls evkalinin/gh-md-toc:0.7.0. Docker Hub has tags up to
  0.10.0, which matches gh_toc_version.
• Tests shows an old run with 6 tests, including "TOC for remote README.md" and
  "TOC for mixed README.md (remote/local)", which are commented out now. The suite
  has 11 tests.
• "Tested on" statements: "macOS High Sierra (gh-md-toc release 0.4.9)" in the
  intro and "Ubuntu 14.04/14.10 in bash/zsh" in Dependency. CI runs on
  ubuntu-latest.
• Combo example has a typo in the URL: sitemap.s instead of sitemap.js.

Notes

• tests/tests.bats asserts README's heading list, so headings should stay
  unchanged.