build!: update library to use Node 12

[alexander-fenster]

Migrate library to Node 12

[generated-files-bot[bot]]

Warning: This pull request is touching the following templated files:

• .kokoro/continuous/node10/common.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/continuous/node10/docs.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/continuous/node10/test.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/continuous/node8/common.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/continuous/node8/test.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/presubmit/node10/common.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/presubmit/node10/docs.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/presubmit/node10/lint.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/presubmit/node10/test.cfg - .kokoro files are templated and should be updated in synthtool
• .github/workflows/ci.yaml - .github/workflows/ci.yaml (GitHub Actions) should be updated in synthtool