Update pin and add pin-slash (#24669)

Continue #23531
Thanks for the update in https://github.com/primer/octicons/issues/940,
@CameronFoxly

![image](https://github.com/go-gitea/gitea/assets/18380374/bc512b20-b656-4bd7-8e70-3a2b7eca9d65)

![image](https://github.com/go-gitea/gitea/assets/18380374/a3863267-6a7a-48d5-8157-9f361fec3fed)

templates/projects/view.tmpl

@@ -109,7 +109,7 @@
 									data-modal-default-project-board-header="{{$.locale.Tr "repo.projects.column.unset_default"}}"
 									data-modal-default-project-board-content="{{$.locale.Tr "repo.projects.column.unset_default_desc"}}"
 									data-url="{{$.Link}}/{{.ID}}/unsetdefault">
-										{{svg "octicon-pin"}}
+										{{svg "octicon-pin-slash"}}
 										{{$.locale.Tr "repo.projects.column.unset_default"}}
 									</a>
 								{{end}}