Fix RFC2119 keyword warnings

[yoshisatoyanagisawa]

The current specification uses "should" and "may" in non-normative sections. They are recognized as RFC2119 keywords. This change omit using them there.

• [X] At least two implementers are interested (and none opposed):
  • Google Chrome
  • Firefox
• [X] Tests are written and can be reviewed and commented upon at:
  • n/a (this is changes in non-normative sections.)
• [X] Implementation bugs are filed:
  • Chromium: n/a (no behavior change is required)
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1855580 (vendor does not yet implement the spec)
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=269893 (vendor does not yet implement the spec)
  • Deno: n/a (no reason to believe a change is required)
  • kenchris/urlpattern-polyfill: n/a (no reason to believe a change is required)
• [X] MDN issue is filed: n/a (change is on a spec detail)
• [X] The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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[jeremyroman]

This probably should be Editorial per https://github.com/whatwg/meta/blob/main/COMMITTING.md. I think I can do that while merging.

[yoshisatoyanagisawa]

thanks for the review!