Deprecate old runtimes

[dyladan]

THIS IS A BREAKING CHANGE. PLEASE MAKE PR TO THE next BRANCH FOR INCLUSION IN THE 2.0 RELEASE

Remove testing and documentation of support for unsupported runtimes and document the following policy:

Old runtimes defined as runtimes no longer supported by NodeJS. When a LTS NodeJS version reaches end of life, we will release a new major version which drops support for it. The previous major version will continue to receive security and p1 bug fixes for 1 year after the new major version release.

[pichlermarc]

We should also remove existing code that soley exists for the purpose of keeping old runtimes alive. See #3689.

[pichlermarc]

once we've drop support for node 14, we'll need to remove the workaround for nx not supporting Node 14 anymore:

opentelemetry-js/.github/workflows/unit-test.yml

Lines 49 to 51 in 415b198

	# temporary fix for https://github.com/nrwl/nx/issues/19022
	- run: npm install --save-dev nx@16.7.4 @nx/devkit@16.7.4
	if: ${{ matrix.node_version == '14' }}

We can then close #4123

[pichlermarc]

We'll also need to drop the node v14 (possibly also v16) workarounds here:

opentelemetry-js/.github/workflows/unit-test.yml

Lines 29 to 34 in 415b198

	- run: npm install -g npm@latest
	if: ${{ matrix.node_version == '18' || matrix.node_version == '20' }}
	# npm@10.0.0 drops support for Node.js v14 and v16
	- run: npm install -g npm@"<10.0.0"
	if: ${{ matrix.node_version == '14' || matrix.node_version == '16' }}