docs: update plan names in docs content (#67573)

## What
<!--
* Describe what the change is solving. Link all GitHub issues related to
this change.
-->

This pull request updates plan names across most of the Documentation so
they're consistent with our current plan names.

## How
<!--
* Describe how code changes achieve the solution.
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My original plan was to convert free text to MDX variables so we only
had to make future updates to names in one place. While broadly
successful, there were numerous edge cases that made rolling this out
almost impossible. There were too many ways and places you couldn't use
variables due to a variety of limitations in Docusaurus and Airbyte's
internal MarkDown processor. Explaining how to properly use them made me
realize how prohibitively insufficient this was. In the end, I opted to
return to using free text for plan names.

Scope is now broadly reduced. This PR:

- Converts remaining instances of old plan names to new plan names. In
most cases, I replaced old plan names with new plan names directly. In
some cases, sentences were rewritten to make a bit more sense or be more
maintainable in the future.

- Removes previously added preprocessor variables from Docusaurus
configuration.

- Update Vale styles or various artifacts of content based on linter
findings.

## Review guide
<!--
1. `x.py`
2. `y.py`
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Spot check updated pages to ensure plan names appear appropriately. It's
probably not necessary to check every single instance in detail.

For Platform docs, changes only apply to the Next/Cloud version. After
merging, I'll regenerate 2.0 docs based on this. 1.8 and before won't be
updated.

## User Impact
<!--
* What is the end result perceived by the user?
* If there are negative side effects, please list them. 
-->

People can see correct plan names in docs content.

## Can this PR be safely reverted and rolled back?
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* If unsure, leave it blank.
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- [x] YES 💚
- [ ] NO ❌

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

docs/platform/deploying-airbyte/chart-v2-community.mdx

@@ -7,7 +7,7 @@ import TabItem from '@theme/TabItem';
 import EnvironmentVarConversion from '@site/static/_extraenv_to_helm_chart_v2.md';
 import HelmSyntaxConversion from '@site/static/_helm-chart-2-syntax-changes.md';
 
-# Upgrade to Helm chart V2 (Self-Managed Community)
+# Upgrade to Helm chart V2 (Core)
 
 Airbyte has upgraded its Helm chart to a new version called "V2." Using Helm chart V2 is currently optional. At some future date the V2 Helm chart will become the standard, so we advise that you upgrade your existing deployment to use the new chart before the transition. If you're a new Airbyte user, you can skip the upgrade altogether and start with the new chart.