Add support for optional completionitem.tags

Add support for optional completionitem.tags

* eglot.el (eglot--lsp-interface-alist): Add optional CompletionItem.tags.
(eglot-completion-at-point): Add :company-deprecated key and value,
checking for either the appropriate tag (1) in the :tags property, or
a truthy value for the :deprecated property.
(eglot-client-capabilities): Advertise tagSupport (for tag == 1) and
deprecatedSupport.

Also load an updated version of seq.el in Emacsen < 27.

* Makefile (ELPA_DEPS): Require latest seq.el.