[meta] Remove IE11 Support from Olivero

I think only 📌 Olivero: Simplification of the grid layouts to use CSS grid (instead of Flexbox) Needs work remains here. Downgrading to major, as there is no problem if we don't complete this, it is only removing some technical debt.

Is an issue needed for this item in the Issue Summary, "Ability to use negative numbers when specifying grid columns.'?

In #16 @longwave suggests that this is complete.

I tried to confirm that pinging the Olivero maintainers in #olivero in November but haven't had a response. I tried also in #core-development in December but didn't get a response there either.

So, are we done here?

Changing status in another attempt to get feedback.

Still several references in the CSS that can likely be removed in a new child issue:

core/themes/olivero/css/components/header-search-narrow.pcss.css:    background-image: linear-gradient(var(--color--primary-50), var(--color--primary-50)); /* Two values are needed for IE11 support. */
core/themes/olivero/css/components/header-search-narrow.pcss.css:      /* IE11's high contrast show will not show the background image, so we show the text. */
core/themes/olivero/css/components/node.css:  background-image: linear-gradient(var(--color--primary-50), var(--color--primary-50)); /* Two values are needed for IE11 support. */
core/themes/olivero/css/components/search-results.pcss.css:    background-image: linear-gradient(var(--color--primary-50), var(--color--primary-50)); /* Two values are needed for IE11 support. */
core/themes/olivero/css/components/button.pcss.css:    IE11 doesn't work properly on button elements so we only do
core/themes/olivero/css/components/header-search-wide.css:  z-index: 1; /* Ensure left border shows above social region in IE11. */
core/themes/olivero/css/components/header-search-wide.css:  background-image: linear-gradient(var(--color--primary-50), var(--color--primary-50)); /* Two values are needed for IE11 support. */
core/themes/olivero/css/components/search-results.css:  background-image: linear-gradient(var(--color--primary-50), var(--color--primary-50)); /* Two values are needed for IE11 support. */
core/themes/olivero/css/components/navigation/nav-primary-no-js.pcss.css:       * because it will break IE11 and earlier versions of MS Edge.
core/themes/olivero/css/components/header-search-wide.pcss.css:  z-index: 1; /* Ensure left border shows above social region in IE11. */
core/themes/olivero/css/components/header-search-wide.pcss.css:  background-image: linear-gradient(var(--color--primary-50), var(--color--primary-50)); /* Two values are needed for IE11 support. */
core/themes/olivero/css/components/navigation/nav-primary-no-js.css:       * because it will break IE11 and earlier versions of MS Edge.
core/themes/olivero/css/components/navigation/nav-primary-wide.pcss.css:        overflow: visible; /* Necessary to view icon in IE11 */
core/themes/olivero/css/components/header-search-narrow.css:  background-image: linear-gradient(var(--color--primary-50), var(--color--primary-50)); /* Two values are needed for IE11 support. */
core/themes/olivero/css/components/header-search-narrow.css:  /* IE11's high contrast show will not show the background image, so we show the text. */
core/themes/olivero/css/components/navigation/nav-primary-wide.css:    overflow: visible; /* Necessary to view icon in IE11 */
core/themes/olivero/css/components/button.css:    IE11 doesn't work properly on button elements so we only do
core/themes/olivero/css/components/node.pcss.css:    background-image: linear-gradient(var(--color--primary-50), var(--color--primary-50)); /* Two values are needed for IE11 support. */

Opened 📌 Remove remaining IE11 support from Olivero Active