Bump version to 21.5.0 #1110

Job	Run time
build (stellar-cli, stellar, ubuntu-20.04, x86_64-unknown-linux-gnu)	14m 30s
build (stellar-cli, stellar, ubuntu-20.04, aarch64-unknown-linux-gnu)	16m 28s
build (stellar-cli, stellar, macos-14, x86_64-apple-darwin)	9m 15s
build (stellar-cli, stellar, macos-12, aarch64-apple-darwin)	14m 33s
build (stellar-cli, stellar, windows-latest, x86_64-pc-windows-msvc, .exe)	21m 15s
build (soroban-cli, soroban, ubuntu-20.04, x86_64-unknown-linux-gnu)	14m 47s
build (soroban-cli, soroban, ubuntu-20.04, aarch64-unknown-linux-gnu)	16m 30s
build (soroban-cli, soroban, macos-14, x86_64-apple-darwin)	9m 14s
build (soroban-cli, soroban, macos-12, aarch64-apple-darwin)	14m 12s
build (soroban-cli, soroban, windows-latest, x86_64-pc-windows-msvc, .exe)	20m 14s
	2h 30m 58s

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