Add the option of making prereleases.

https://semver.org/spec/v2.0.0.html specifies how these look like. We allow just appending "-[0-9a-z]*". (So e.g we can add -pre0 to the version).

Also change the regexes to actually require a "." instead of allowing an arbitrary character.

PiperOrigin-RevId: 816701210
Change-Id: Ia55e89bb82d1084f6788699f620f1fed1da8549a

Author: tholenst

kokoro/testutils/github_release_util.sh

@@ -84,9 +84,8 @@ process_params() {
   readonly ACTION
 
   VERSION="$2"
-  if [[ ! "${VERSION}" =~ ^[0-9]+.[0-9]+.[0-9]+$ ]]; then
-    echo "ERROR: Invalid version format: expected MAJOR.MINOR.PATCH, got \
-${VERSION}" >&2
+  if [[ ! "${VERSION}" =~ ^[0-9]+\.[0-9]+\.[0-9]+(-[0-9a-z]*)?$ ]]; then
+    echo "ERROR: Invalid version format: expected MAJOR.MINOR.PATCH or MAJOR.MINOR.PATCH-pre, got \${VERSION}" >&2
     usage
   fi
   readonly VERSION