Add 'stable' to the list of infinity version names. (#28772)

* Add 'stable' to the list of infinity version names.
Rename libunwind 1.5-head to 1.5-stable.

* Add stable to the infinite version list in packaging_guide.rst.

lib/spack/docs/packaging_guide.rst

@@ -705,7 +705,8 @@ as follows:
 
 #. The following special strings are considered larger than any other
    numeric or non-numeric version component, and satisfy the following
-   order between themselves: ``develop > main > master > head > trunk``.
+   order between themselves:
+   ``develop > main > master > head > trunk > stable``.
 
 #. Numbers are ordered numerically, are less than special strings, and
    larger than other non-numeric components.

lib/spack/spack/version.py

@@ -59,7 +59,7 @@
                           "(?:[+][0-9A-Za-z-]+)?)")
 
 # Infinity-like versions. The order in the list implies the comparison rules
-infinity_versions = ['develop', 'main', 'master', 'head', 'trunk']
+infinity_versions = ['develop', 'main', 'master', 'head', 'trunk', 'stable']
 
 iv_min_len = min(len(s) for s in infinity_versions)