Docs: Improve v8 upgrade notes for SQL data sources

[marefr]

What this PR does / why we need it:
Improves SQL data sources documentation in regards to Grafana 8 changes and adds some additional upgrade notes in regards to this.

Which issue(s) this PR fixes:
Ref #35390
Ref #38666

Special notes for your reviewer:

[torkelo]

Might be worth mentioning that string fields get transformed into labels when time series format it selected. And an example of how you only show a specific label using the display name variable expression.

[achatterjee-grafana]

Added partial doc review, more to come.

[torkelo]

I think this looks great

[marefr]

Changed MSSQL quite a bit to make its examples more in line with Postgres and MSSQL and by that removed some screenshots.

[achatterjee-grafana]

Added doc review.

[achatterjee-grafana]

LGTM

[grafanabot]

The backport to v8.0.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-38792-to-v8.0.x origin/v8.0.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 0bb7d50ad62181544a001b43ea0f8e7935548095
# Push it to GitHub
git push --set-upstream origin backport-38792-to-v8.0.x
git switch main
# Remove the local backport branch
git branch -D backport-38792-to-v8.0.x

Then, create a pull request where the base branch is v8.0.x and the compare/head branch is backport-38792-to-v8.0.x.

[grafanabot]

The backport to v8.1.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-38792-to-v8.1.x origin/v8.1.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 0bb7d50ad62181544a001b43ea0f8e7935548095
# Push it to GitHub
git push --set-upstream origin backport-38792-to-v8.1.x
git switch main
# Remove the local backport branch
git branch -D backport-38792-to-v8.1.x

Then, create a pull request where the base branch is v8.1.x and the compare/head branch is backport-38792-to-v8.1.x.