Update indexmap, bump MSRV #28

Job	Run time
Run tests (macos-latest, stable)	17s
Run tests with Miri (macos-latest)	6m 16s
Run tests (macos-latest, beta)	24s
Run tests with Miri (ubuntu-latest)	6m 14s
Run tests with Miri (windows-latest)	5m 56s
Run tests (macos-latest, nightly)	23s
Run tests (macos-latest, 1.56.0)	3m 55s
Run tests (ubuntu-latest, stable)	18s
Run tests (ubuntu-latest, beta)	16s
Run tests (ubuntu-latest, nightly)	24s
Run tests (ubuntu-latest, 1.56.0)	3m 36s
Run tests (windows-latest, stable)	1m 44s
Run tests (windows-latest, beta)	46s
Run tests (windows-latest, nightly)	58s
Run tests (windows-latest, 1.56.0)	3m 40s
	35m 7s

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