That link you provided pointed me (thank you) in the right direction for a DOWNGRADE - so others beware.
Here's what I did to fix it as it looks like Microsoft it building against Ubuntu 18.04 now (which is probably a different version of GLIBC)
Microsoft says to do an "apt-get install mssql-server=15.0.4063.15-10" in order to downgrade to August 2020 version CU6 - but this didn't do anything for me.
Instead I downloaded the DEB package built against 16.04 LTS and installed it like the following...(thankfully they are building against 16.04!!!!)
apt install ./mssql-server_15.0.4063.15-10_amd64.deb
Note, selecting 'mssql-server' instead of './mssql-server_15.0.4063.15-10_amd64.deb'
The following packages will be DOWNGRADED:
0 upgraded, 0 newly installed, 1 downgraded, 0 to remove and 0 not upgraded.
Need to get 0 B/222 MB of archives.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] Y