Update PRs & Issue templates to exclude 3.4 from active development.

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -15,12 +15,12 @@ body:
       Please provide the following system information to help us diagnose the bug. For example:
 
       // example for c++ user
-      OpenCV version: 4.6.0
+      OpenCV version: 4.8.0
       Operating System / Platform: Ubuntu 20.04
       Compiler & compiler version: GCC 9.3.0
 
       // example for python user
-      OpenCV python version: 4.6.0.66
+      OpenCV python version: 4.8.0.74
       Operating System / Platform: Ubuntu 20.04
       Python version: 3.9.6
   validations:

.github/ISSUE_TEMPLATE/documentation.yml

@@ -12,7 +12,7 @@ body:
   attributes:
     label: Describe the doc issue
     description: >
-      Please provide a clear and concise description of what content in https://docs.opencv.org/ is an issue. Note that there are multiple active branches, such as 3.4, 4.x and 5.x, so please specify the branch with the problem.
+      Please provide a clear and concise description of what content in https://docs.opencv.org/ is an issue. Note that there are multiple active branches, such as 4.x and 5.x, so please specify the branch with the problem.
     placeholder: |
       A clear and concise description of what content in https://docs.opencv.org/ is an issue.