Bump minimum Node.js version to 16

[rekmarks]

This PR bumps the minimum Node.js version of the extension to ^16. I had to change a couple of expected error messages in tests, but otherwise everything appears to work out of the box.

The motivations for this are as follows:

• The Snaps team is facing some issues with Node 14 in our monorepo, and rather than hacking our way to a solution we'd rather abandon Node 14.
• We have to do this in less than a year anyway, due to the Node.js LTS schedule.
• Node 16 is probably a little bit faster than 14, so everybody wins when we do this.

[metamaskbot]

Builds ready [7a0e974]

• builds: chrome, firefox, opera
• builds (beta): chrome, firefox, opera
• builds (flask): chrome, firefox, opera
• build viz: Build System
• code coverage: Report
• storybook: Storybook
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5, 6, 7, 8
  • common: 0, 1, 2, 3, 4, 5, 6, 7, 8
  • content-script: 0
  • contentscript: contentscript
  • disable-console: disable-console
  • inpage: inpage
  • policy-load: policy-load
  • sentry-install: sentry-install
  • ui: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9

Page Load Metrics (1656 ± 34 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	82	139	97	13	6
		domContentLoaded	1529	1817	1638	70	33
		load	1564	1834	1656	71	34
		domInteractive	1529	1817	1638	70	33

[metamaskbot]

Builds ready [cf0e1c3]

• builds: chrome, firefox, opera
• builds (beta): chrome, firefox, opera
• builds (flask): chrome, firefox, opera
• build viz: Build System
• code coverage: Report
• storybook: Storybook
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5, 6, 7, 8
  • common: 0, 1, 2, 3, 4, 5, 6, 7, 8
  • content-script: 0
  • contentscript: contentscript
  • disable-console: disable-console
  • inpage: inpage
  • policy-load: policy-load
  • sentry-install: sentry-install
  • ui: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9

Page Load Metrics (1614 ± 33 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	81	177	98	20	10
		domContentLoaded	1522	1709	1598	53	26
		load	1522	1769	1614	69	33
		domInteractive	1522	1709	1598	53	26