build(java): use actions/github-script v3 tag rather than full semver (#521)

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

Source-Author: Stephanie Wang <[email protected]>
Source-Date: Tue Nov 17 13:02:02 2020 -0500
Source-Repo: googleapis/synthtool
Source-Sha: 7fcc405a579d5d53a726ff3da1b7c8c08f0f2d58
Source-Link: googleapis/synthtool@7fcc405

Author: yoshi-automation

.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

@@ -11,7 +11,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-bigquerydatatransfer.git",
-        "sha": "6eeec2ff5a114245dca2da7295d5008e38c504f5"
+        "sha": "76c340e927a793d7324eb6c004fe452d1b6cfd1c"
       }
     },
     {
@@ -26,7 +26,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "7d652819519dfa24da9e14548232e4aaba71a11c"
+        "sha": "7fcc405a579d5d53a726ff3da1b7c8c08f0f2d58"
       }
     }
   ],