upgrade node version to 18

[milaGGL]

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Fixes #<issue_number_goes_here> 🦕

[generated-files-bot[bot]]

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

• .kokoro/continuous/node14/common.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/continuous/node14/lint.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/continuous/node14/samples-test.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/continuous/node14/system-test.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/continuous/node14/test.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/presubmit/node14/common.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/presubmit/node14/samples-test.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/presubmit/node14/system-test.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/presubmit/node14/test.cfg - .kokoro files are templated and should be updated in synthtool