switch to calver; add g:vimwiki_version; tidy version hardcoding

Setting a global for g:vimwiki_version doesn't seem too egregious, and
lets us reference it cleanly elsewhere.

Removes version header from vimwiki.txt and hardcoding in tagfile
generation.

There's still one instance of that left in test/tag.vader that we should
clean up eventually.

doc/vimwiki.txt

@@ -8,9 +8,6 @@
              |     | |   | | ||_|| ||   _   ||   | |    _  ||   |            ~
               |___|  |___| |_|   |_||__| |__||___| |___| |_||___|            ~
 
-
-                               Version: 2.6.0
-
 ==============================================================================
 CONTENTS                                                             *vimwiki*
 
@@ -3995,14 +3992,17 @@ https://github.com/vimwiki/vimwiki/issues/, all others from
 http://code.google.com/p/vimwiki/issues/list. They may be accessible from
 https://github.com/vimwiki-backup/vimwiki/issues.
 
-From version 2.6.0, the VimWiki project has adopted a rolling release policy.
-Once changes are accepted, they will merge directly to dev, which is now the
-main branch.  master is retained as a legacy mirror of the dev branch.
+From version 2022.12.02, what would have been 2.6.0 under the prior versioning
+scheme, the VimWiki project has adopted a rolling release policy.  Once changes
+are accepted, they will merge directly to dev, which is now the main branch.
+master is retained as a legacy mirror of the dev branch.
 
-2.6.0 (2022-11-28)~
+This is somewhat experimental, and will probably be refined over time.
+
+2022.12.02~
 
 New:~
-    * Policy: #1235: Move to semver and rolling release cadence
+    * Policy: #1235: Move to calendar versioning and rolling release cadence
     * Feature: #238: Reuse existing tabs with tab drop
     * Issue #621: Feature request: Highlight code listings in HTML
     * Issue #290: Calendar plugin, do not sign if no wiki