chore: Increase required Pillow version

[Dosenpfand]

Description

1. Increase the required Pillow version, as 7.x contains multiple vulnerabilities.
2. Remove the mention of the unmaintained PIL from installation.rst.

ADDITIONAL INFORMATION

• Has associated issue:
• Is CRUD MVC related.
• Is Auth, RBAC security related.
• Changes the security db schema.
• Introduces new feature
• Removes existing feature

[codecov]

Codecov Report

Merging #1873 (dd57b4b) into master (cc8e359) will decrease coverage by 32.75%.
The diff coverage is n/a.

❗ Current head dd57b4b differs from pull request most recent head 3ba003c. Consider uploading reports for the commit 3ba003c to get more accurate results

@@             Coverage Diff             @@
##           master    #1873       +/-   ##
===========================================
- Coverage   77.67%   44.91%   -32.76%     
===========================================
  Files          70       70               
  Lines        8560     8420      -140     
===========================================
- Hits         6649     3782     -2867     
- Misses       1911     4638     +2727     

Flag	Coverage Δ
python	44.91% <ø> (-32.76%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flask_appbuilder/security/sqla/apis/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
flask_appbuilder/security/sqla/apis/role/schema.py	0.00% <0.00%> (-100.00%)	⬇️
flask_appbuilder/security/sqla/apis/user/schema.py	0.00% <0.00%> (-100.00%)	⬇️
...ask_appbuilder/security/sqla/apis/role/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
...ask_appbuilder/security/sqla/apis/user/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
...ask_appbuilder/security/sqla/apis/view_menu/api.py	0.00% <0.00%> (-100.00%)	⬇️
...sk_appbuilder/security/sqla/apis/permission/api.py	0.00% <0.00%> (-100.00%)	⬇️
...sk_appbuilder/security/sqla/apis/user/validator.py	0.00% <0.00%> (-100.00%)	⬇️
...ppbuilder/security/sqla/apis/view_menu/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
...pbuilder/security/sqla/apis/permission/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
... and 46 more

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