fix csi500 end date issue

[qianyun210603]

Description

Use today instead of returned date from baostock as the end date of csi500 instruments.

Motivation and Context

The "end_date" of get_new_companies should be today instead of the returned date of baostock.query_zz500_stocks, as the latter is the latest update date before the date given in parameter, it is not the last effective date thus does not change with the data date.
As a result, running the current version (with bug) today returns csi500 instrument file as below

SH600006	2013-06-25	2022-08-01
SH600008	2018-12-11	2022-08-01
SH600021	2018-06-05	2022-08-01
...

(to verify, running qlib\scripts\data_collector\cn_index>python collector.py --qlib_dir E:\fake1ddir --index-name=CSI500)
and any test with stock population of csi500 ends at 2022-08-01.

This pull request fixes the bug.

How Has This Been Tested?

• Pass the test by running: pytest qlib/tests/test_all_pipeline.py under upper directory of qlib.
• If you are adding a new feature, test on your own test scripts.

Screenshots of Test Results (if appropriate):

1. Pipeline test:
2. Your own tests:
   running the collector generates correct instrument population file

SH600006	2013-06-25	2022-11-22
SH600008	2018-12-11	2022-11-22
SH600021	2018-06-05	2022-11-22
SH600022	2016-12-06	2022-11-22
SH600026	2013-06-25	2022-11-22
SH600027	2021-06-16	2022-11-22

Types of changes

• Fix bugs
• Add new feature
• Update documentation

[you-n-g]

Thanks so much!