[confmap] Set confmap.unifyEnvVarExpansion feature gate to stable (#10508)

#### Description

This PR promotes the `confmap.unifyEnvVarExpansion` feature gate to
stable and sets a `ToVersion` of `v0.106.0`, anticipating that the gate
be completely removed in that version.

We should weigh if switching the Stable should be done in `v0.105.0` or
if it needs more time in `Beta` to give users more time to switch.
Delaying promotion to `Stable` delays confmap 1.0.

If we merge this we need to commit to merging
#10510 in
the same release.

#### Link to tracking issue
Related to
#10161
Related to
#7111
Related to
#8215

---------

Co-authored-by: Evan Bradley <[email protected]>

Author: TylerHelmuth

.chloggen/unify-env-var-gate-stable.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: breaking
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: confmap
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Set the `confmap.unifyEnvVarExpansion` feature gate to Stable. Expansion of `$FOO` env vars is no longer supported.  Use `${FOO}` or `${env:FOO}` instead.
+
+# One or more tracking issues or pull requests related to the change
+issues: [10508]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []