backend: update stage dates

* source/compiler_stages/tc_5/tc_5.rst,
* source/compiler_stages/tc_6/tc_6.rst,
* source/compiler_stages/tc_7/tc_7.rst,
* source/compiler_stages/tc_8/tc_8.rst,
* source/intro/history/tiger_2022.rst: Here.
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00:00:41