upgrade to go1.21, stop directly relying on golang.org/x/exp #65

	name: on-push
	on: [push]
	jobs:
	test:
	name: Build
	runs-on: ubuntu-latest
	steps:
	- name: Checkout
	uses: actions/checkout@v3
	- name: Setup Go
	uses: actions/setup-go@v3
	with:
	go-version: '1.21'
	- name: Generate lexer/parser files with antlr
	run: ./generateParser.sh
	- name: Display potential updates
	run: git diff
	- name: Make sure lexer/parser files were already up-to-date
	run: git diff-index --quiet HEAD
	- name: Test
	run: go test -coverprofile='coverage.txt' -covermode=atomic ./...
	- name: Upload coverage
	uses: codecov/codecov-action@v3
	with:
	files: ./coverage.txt
	token: ${{ secrets.CODECOV_TOKEN }}

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