Update bump.sh to go SNAPSHOT to version

This updates the release script to go from -SNAPSHOT to its next version automatically.

From 4.3.0-SNAPSHOT:

Then, from 4.3.0 to 4.3.1-SNAPSHOT:

When I (or anyone else) gets time, maybe we could add some bats tests for bump.sh.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

cc @OpenAPITools/build

bin/utils/release/bump.sh

@@ -163,11 +163,13 @@ if [[ -z "${to}" ]]; then
             ;;
         snapshot)
             if [[ true = ${from_parts[3]} ]]; then
-                err "Can't move from SNAPSHOT to SNAPSHOT (from=${from})."
+                # Going from -SNAPSHOT to its release
+                to="${from_parts[0]}.${from_parts[1]}.${from_parts[2]}"
             else
+                # Going from some version to its next version and -SNAPSHOT
                 to="${from_parts[0]}.${from_parts[1]}.$(( ${from_parts[2]} + 1 ))-SNAPSHOT"
-                version "$to" to_parts
             fi
+            version "$to" to_parts
             ;;
     esac
    fi