bump TF version to 2.13 #353

Job	Run time
ubuntu-latest, tf-default, R-release	9m 51s
windows-latest, tf-default, R-release	17m 46s
macOS-latest, tf-default, R-release	15m 38s
ubuntu-latest, tf-default, R-oldrel	13m 45s
windows-latest, tf-default, R-oldrel	22m 0s
macOS-latest, tf-default, R-oldrel	22m 0s
ubuntu-latest, tf-default, R-oldrel-1	12m 30s
ubuntu-latest, tf-default, R-3.6	11m 32s
ubuntu-20.04, tf-release, R-release	13m 22s
windows-latest, tf-release, R-release	20m 57s
macOS-latest, tf-release, R-release	16m 37s
ubuntu-latest, tf-2.13, R-release	10m 20s
ubuntu-latest, tf-2.12, R-release	13m 46s
ubuntu-latest, tf-2.11, R-release	9m 52s
ubuntu-latest, tf-2.10, R-release	13m 56s
ubuntu-latest, tf-2.8, R-release	12m 53s
	3h 56m 45s

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