ci: keep release-please in 0.x with conservative bumps

Set bump-minor-pre-major and bump-patch-for-minor-pre-major so that
while the crate is pre-1.0, breaking changes bump the minor version
(not to 1.0.0) and ordinary features bump the patch version. Only
breaking changes move the minor digit.

Author: dstoc

release-please-config.json

@@ -1,5 +1,7 @@
 {
   "$schema": "https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json",
+  "bump-minor-pre-major": true,
+  "bump-patch-for-minor-pre-major": true,
   "packages": {
     ".": {
       "release-type": "rust"