Add env var syntax deprecation note to 12 OpenTelemetry app docs

[sumoanema]

Purpose of this pull request

Adding a note for 12 opentelemetry app docs, after deprecation of env variable syntax in opentelemetry collector syntax

Select the type of change

• Minor Changes - Typos, formatting, slight revisions
• Update Content - Revisions, updating sections
• New Content - New features, sections, pages, tutorials
• Site and Tools - .clabot, version updates, maintenance, dependencies, new packages for the site (Docusaurus, Gatsby, React, etc.)

Ticket (if applicable)

[kimsauce]

Hi @sumoanema , many thanks for your PR! I converted your note into a reusable reference, and then insert it in each doc. Does this look OK?

[sumoanema]

@kimsauce - So there are certain apps for which username and password are optional, these apps are JMX, activeMQ, cassandra, apache tomcat .
Only in case do we want the note to have the starting as :
"If you're using a username and password with OpenTelemetry Collector"

for the rest it should be :
"For OpenTelemetry collector version >= 0.104 we recommend to use the downloaded config with environment variable as :
${environment_variable}"

What do you suggest, how should we make changes for it ?

[kimsauce]

@kimsauce - So there are certain apps for which username and password are optional, these apps are JMX, activeMQ, cassandra, apache tomcat . Only in case do we want the note to have the starting as : "If you're using a username and password with OpenTelemetry Collector"

for the rest it should be : "For OpenTelemetry collector version >= 0.104 we recommend to use the downloaded config with environment variable as : ${environment_variable}"

What do you suggest, how should we make changes for it ?

Hi @sumoanema , thank you for clarifying! Ok, I've created two separate reusable versions of the note (required and optional). And put the optional note in JMX, Cassandra, ActiveMQ, Apache Tomcat. The rest have the required version. Can you take a look?

[sumoanema]

Looks good to me @kimsauce

[kimsauce]

Looks good to me @kimsauce

OK! just approved

[JV0812]

LGTM