docs: conf.py: fix cve extlinks caption for sphinx <4.0

extlinks captions support using %s substitution but only from sphinx 4.0
onwards.
c.f. https://www.sphinx-doc.org/en/master/usage/extensions/extlinks.html#confval-extlinks

Weirdly enough, on older sphinx versions, the caption is just a prefix
to the actual text passed to the extlink. Therefore, in that specific
case, CVE- or CVE-%s are identical in meaning for sphinx >=4.0 and since
only CVE- caption works on sphinx <4.0, let's go with CVE- caption
prefix.

Fixes: b311070d866cf "manuals: add 3.4 and 3.4.1 release notes after migration information"
Cc: Quentin Schulz <foss+yocto@0leil.net>
(From yocto-docs rev: c9922076f5c1285d9cfd6aff8ce5b6635d88222f)

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/conf.py

@@ -90,7 +90,7 @@ rst_prolog = """
 
 # external links and substitutions
 extlinks = {
-    'cve': ('https://nvd.nist.gov/vuln/detail/CVE-%s', 'CVE-%s'),
+    'cve': ('https://nvd.nist.gov/vuln/detail/CVE-%s', 'CVE-'),
     'yocto_home': ('https://www.yoctoproject.org%s', None),
     'yocto_wiki': ('https://wiki.yoctoproject.org/wiki%s', None),
     'yocto_dl': ('https://downloads.yoctoproject.org%s', None),