Upgrade postgres 11 to 15 and postgis 2.5 to 3.3

Hello all,

I would like some pointers of how to go about this.
My prod server runs centos 8.2 and is obviously out of any support.
So I decided to install redhat 8.4 with license on a server dedicated for this conversion.
Soon after I realize that postgres 11 and postgis 2.5 is no longer obtainable to be installed from Postgres.
I realized soon that:

* Postgres 11 seems easy to obtain from unsupported archive sites. Have not tested install yet. May not be 100% good.
* Postgis 2.5 with all dependant packages is not obtainable from unsupported archive sites.

Looking at this from above, what do I do here? I cannot be the only one facing this dead end road, right?
So far my effort have been on Redhat Linux.
Are there other Linux variants where this conversion can take place easily?
Since this is a just a conversion, Im open to even get this done on Windows.

What would be good plan here?

Best,
Herman