ci: Switch OIIO dev-2.3 branch to using v2.3.21.0 tag (#1701)

OIIO's 2.3 family is obsolete and no longer updating, so we removed
the branch marker. (Branch markers should always indicate places where
development may happen.)

But we still reference that named branch in a couple places for OSL's CI,
so just replace it with the immutable tag of the last release in the 2.3
family.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

.github/workflows/ci.yml

@@ -87,7 +87,7 @@ jobs:
             container: aswftesting/ci-osl:2021-clang11
             vfxyear: 2021
             cxx_std: 17
-            openimageio_ver: dev-2.3
+            openimageio_ver: v2.3.21.0
             python_ver: 3.7
             pybind11_ver: v2.7.0
             simd: avx2,f16c
@@ -306,7 +306,7 @@ jobs:
             cxx_compiler: g++-7
             cxx_std: 14
             openexr_ver: v2.4.3
-            openimageio_ver: dev-2.3
+            openimageio_ver: v2.3.21.0
             pybind11_ver: v2.6.2
             python_ver: 2.7
             simd: sse4.2