Bump rust version for cross-compatibility tests #4487

Job	Run time
check docs (ubuntu-latest / stable)	1m 1s
check examples (ubuntu-latest / stable)	51s
check feature flags (ubuntu-latest / stable)	15m 55s
check format (ubuntu-latest / nightly)	16s
check licenses (ubuntu-latest / stable)	29s
check linter (ubuntu-latest / stable)	1m 26s
check minimal versions of transitive dependencies (ubuntu-latest / stable)	31s
check minimal supported rust version (ubuntu-latest / msrv)	26s
check spelling (ubuntu-latest / nightly)	5m 42s
test (ubuntu-latest / stable)	8m 2s
test (macos-latest / stable)	9m 20s
test (windows-latest / stable)	5m 42s
	49m 41s

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