build(java): use actions/github-script v3 tag rather than full semver

So that renovate-bot stops proposing PRs like [this](https://github.com/googleapis/java-bigquerydatatransfer/pull/515/files)!

Source-Author: Stephanie Wang <stephaniewang526@users.noreply.github.com>
Source-Date: Tue Nov 17 13:02:02 2020 -0500
Source-Repo: googleapis/synthtool
Source-Sha: 7fcc405a579d5d53a726ff3da1b7c8c08f0f2d58
Source-Link: googleapis/synthtool@7fcc405

.github/workflows/approve-readme.yaml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     if: github.repository_owner == 'googleapis' && github.head_ref == 'autosynth-readme'
     steps:
-      - uses: actions/github-script@v3.0.0
+      - uses: actions/github-script@v3
         with:
           github-token: ${{secrets.YOSHI_APPROVER_TOKEN}}
           script: |

.github/workflows/auto-release.yaml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     if: contains(github.head_ref, 'release-v')
     steps:
-    - uses: actions/github-script@v3.0.0
+    - uses: actions/github-script@v3
       with:
         github-token: ${{secrets.YOSHI_APPROVER_TOKEN}}
         debug: true

synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-container.git",
-        "sha": "e6e61ada17d6efd52f4e14f50b4005f8b43e7d93"
+        "sha": "4ced51abb40d6ecaa45e0c8474bf52b604665f6b"
       }
     },
     {
@@ -19,7 +19,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "e89175cf074dccc4babb4eca66ae913696e47a71"
+        "sha": "7fcc405a579d5d53a726ff3da1b7c8c08f0f2d58"
       }
     }
   ],