gopls: record that v0.15 will be the last to support go1.18

...and v0.14.2 is the last to do so without warnings.

Change-Id: Ie8cfd5e0337632fb60900938667e0a2a682ecb65
Reviewed-on: https://go-review.googlesource.com/c/tools/+/562682
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: adonovan

gopls/README.md

@@ -94,6 +94,7 @@ version of gopls.
 | Go 1.12     | [[email protected]](https://github.com/golang/tools/releases/tag/gopls%2Fv0.7.5) |
 | Go 1.15     | [[email protected]](https://github.com/golang/tools/releases/tag/gopls%2Fv0.9.5) |
 | Go 1.17     | [[email protected]](https://github.com/golang/tools/releases/tag/gopls%2Fv0.11.0) |
+| Go 1.18     | [[email protected]](https://github.com/golang/tools/releases/tag/gopls%2Fv0.14.2) |
 
 Our extended support is enforced via [continuous integration with older Go
 versions](doc/contributing.md#ci). This legacy Go CI may not block releases: