Branch and tag reorganization

[sbrunner]

We need to update the used GDAL version on stabilization branches and to do that smoothly we need to reorganize the branches and tags.

The branches were renamed as the following:

• 7.6 => 7.6-ubuntu18.04
• 7.6-20_04 => 7.6-gdal3.3
• 8.0 => 8.0-gdal3.6

And they are published on the following tags:

• 7.6-ubuntu18.04 => 7.6-ubuntu18.04 and 7.6
• 7.6-gdal3.3 => 7.6-gdal3.3 and 7.6-20_04
• 8.0-gdal3.6 => 8.0-gdal3.6, 8.0 and latest

In a bit more than 2 weeks we will add a version 3.7 of GDAL then the tags 7.6 and 8.0 will be upgraded using a new version of GDAL image, and for the 7.6 that implies that the used version of Ubuntu will also be updated.

In the future, all the short tags like 8.0 can get a new version of GDAL and Ubuntu.

Then, if you don't want that the used version of GDAL or Ubuntu is updated, you should use the new long tag names (like 8.0-gdal3.6).

The currently used tagging schema is visible in the SECURITY.md file of the master branch.

[sbrunner]

Followup:

The tags 7.6-gdal3.7 and 8.0-gdal3.7 but not yet used as 7.6 and respectively 8.0 they will be in the week 43 (from Monday, October 23)

[sbrunner]

Followup:

The changes are just done as expected.