docs: remove core18 references outside of release information

[munlicode]

Fixes #6270

Removed references to core18, and replaced core18 to core22 in how-to and references.

NOTE

• In /docs/how-to/extensions/list-extensions.rst, I replaced snippet with snippet of actual output of snapcraft extensions.
• These files were not changed. They seem to reference to release information. /docs/how-to/change-bases/change-from-core18-to-core20.rst, /docs/how-to/change-bases/index.rst, /docs/_static/js/support-chart.js, /docs/index.rst

---

• I've followed the contribution guidelines.
• I've signed the CLA.
• I've successfully run make lint && make test.
• I've added or updated any relevant documentation.
• In documents I changed, I added a meta description if one was missing.
• I've updated the relevant release notes.

[munlicode]

I also removed references to core18 in /docs/reference/bases.rst. Is it correct, or it should be as it was?

[munlicode]

In how-to and other guides I replaced core18 with core22, but I have not performed verification, because it seemed to change a lot if I did change for it to work. Is there anything to do with it?

[munlicode]

/docs/_static/js/support-chart.js seems to be missing information about release of v9.