fix(core): Stop abusing the cache for avatar upload

[provokateurin]

Extracted from #54803
Instead of using the cache to store the temporary avatar (in case it is not square for cropping), the image is just returned as a base64 URI.
The cropping was also done by the frontend already, so the endpoint for cropping in the backend is not needed anymore either, thus removing the necessity of using the cache completely.

[provokateurin]

/backport to stable32

[provokateurin]

/backport to stable31

[provokateurin]

/backport to stable30

[CarlSchwan]

Nice cleanup 👏

[backportbot]

The backport to stable30 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable30
git pull origin stable30

# Create the new backport branch
git checkout -b backport/55134/stable30

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick bfb4fe8f dfa9fce4

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/55134/stable30

Error: Failed to push branch backport/55134/stable30: remote: {"auth_status":"auth_error","body":"Invalid username or token. Password authentication is not supported for Git operations."}
fatal: Authentication failed for 'https://github.com/nextcloud/server.git/'

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[provokateurin]

/backport to stable32

[provokateurin]

/backport to stable31

[backportbot]

The backport to stable32 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable32
git pull origin stable32

# Create the new backport branch
git checkout -b backport/55134/stable32

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick bfb4fe8f dfa9fce4

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/55134/stable32

Error: Failed to create pull request: Validation Failed: {"resource":"PullRequest","code":"custom","message":"A pull request already exists for nextcloud:backport/55134/stable32."} - https://docs.github.com/rest/pulls/pulls#create-a-pull-request

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[provokateurin]

/backport to stable30

[backportbot]

The backport to stable31 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable31
git pull origin stable31

# Create the new backport branch
git checkout -b backport/55134/stable31

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick bfb4fe8f dfa9fce4

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/55134/stable31

Error: Failed to create pull request: Validation Failed: {"resource":"PullRequest","code":"custom","message":"A pull request already exists for nextcloud:backport/55134/stable31."} - https://docs.github.com/rest/pulls/pulls#create-a-pull-request

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.