Add upgrade doc to v1.12

[allenxu404]

Thank you for contributing to Velero!

Please add a summary of your change

• Add upgrade-to-1.12 doc

Does your change fix a particular issue?

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• Updated the corresponding documentation in site/content/docs/main.

[allenxu404]

/kind changelog-not-required

[allenxu404]

/kind changelog-not-required

[codecov-commenter]

Codecov Report

Merging #6625 (f730362) into main (5f463c5) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is n/a.

❗ Current head f730362 differs from pull request most recent head e070890. Consider uploading reports for the commit e070890 to get more accurate results

@@           Coverage Diff           @@
##             main    #6625   +/-   ##
=======================================
  Coverage   60.17%   60.17%           
=======================================
  Files         242      242           
  Lines       25679    25679           
=======================================
  Hits        15452    15452           
  Misses       9150     9150           
  Partials     1077     1077           

[kaovilai]

Suggested change

	velero-plugin-for-aws=velero/velero-plugin-for-aws:v1.8.0 \
	velero-plugin-for-aws=velero/velero-plugin-for-aws:v1.7.1 \

double checked the tags, don't see v1.8 anywhere. Could you help me understand where you got 1.8 version from?

[kaovilai]

https://hub.docker.com/r/velero/velero-plugin-for-aws/tags

[sseago]

@kaovilai 1.7 was released with velero 1.11. 1.8 will be released with 1.12 (Velero 1.12.0 doesn't exist yet either)

[kaovilai]

from discussions I've received with a maintainer, bumping Y version in plugins is typical.