feat: improve release note generation

[Diaphteiros]

What this PR does / why we need it:
Improves the release note generation in the following ways:

• Removes the duplicate # in front of PR numbers.
• If the PR info contains multiple release note blocks, they are now rendered correctly.
• PRs with empty release note blocks or NONE now don't show up in the release notes anymore (this e.g. usually includes the release PRs).
• The release note generation script is more verbose, which is useful for debugging.
• Fixed a bug in the PR number collection that would occur if the release note generation was not run against the main branch's latest commit (usually not relevant for release note generation, but annoying when trying to test the script).

Which issue(s) this PR fixes:
Fixes openmcp-project/backlog#243

Special notes for your reviewer:
Disclaimer: I tested this locally, but there is not really a way to check whether it also works in the GHA until the PR is merged ...

Here is an example comparison between the old and the new generation logic. It is based on the diff between versions v0.2.0 and v0.3.0 of the Gardener ClusterProvider, since the corresponding PRs contained an example with multiple release note blocks.

The first example was generated with the old logic, the second one with the new logic. They are separated by horizontal lines to be distinguishable from the rest of the PR info.

Note that the old logic example actually contains an unclosed code block, which I had to close (by adding three backticks to the end of the release notes) to not mess up the formatting.

---

Changes included in v0.3.0:

🚀 Features

• feature operator Shoot field overwrite ##68: It is now possible to overwrite any field of the generated shoot manifest by using ClusterConfig resources.

```bugfix user
Fixed a bug in the `AccessRequest` controller that could cause a panic when referencing a `Cluster` resource that does not have a corresponding Gardener shoot.


### 🔧 Chores
- breaking user common api [##72](https://github.com/openmcp-project/cluster-provider-gardener/pull/72): Refactored the operator to fit our common api.

The Gardener ClusterProvider now provides error messages in form of conditions on the different resources.
- renovates runs 'task generate' after upgrade [##66](https://github.com/openmcp-project/cluster-provider-gardener/pull/66).
- renovate minimumReleaseAge back to 0 days [##61](https://github.com/openmcp-project/cluster-provider-gardener/pull/61).
- other dependency renovate updates go lang version [##59](https://github.com/openmcp-project/cluster-provider-gardener/pull/59): automatic golang version update with renovate
- docs user update README [##56](https://github.com/openmcp-project/cluster-provider-gardener/pull/56): Updated the README.
- feature user enable shoot-oidc-extension for all created shoots [##53](https://github.com/openmcp-project/cluster-provider-gardener/pull/53): The Gardener OIDC Extension is now enabled for all created shoots.

---

Changes included in v0.3.0:

🚀 Features

• Shoot field overwrite #68
  • [OPERATOR][FEATURE] It is now possible to overwrite any field of the generated shoot manifest by using ClusterConfig resources.
  • [USER][BUGFIX] Fixed a bug in the AccessRequest controller that could cause a panic when referencing a Cluster resource that does not have a corresponding Gardener shoot.

🔧 Chores

• common api #72
  • [OPERATOR][FEAT] The Gardener ClusterProvider now provides error messages in form of conditions on the different resources.
  • [USER][BREAKING] Refactored the operator to fit our common api.
• renovate updates go lang version #59
  • [DEPENDENCY][OTHER] automatic golang version update with renovate
• update README #56
  • [USER][DOCS] Updated the README.
• enable shoot-oidc-extension for all created shoots #53
  • [USER][FEATURE] The Gardener OIDC Extension is now enabled for all created shoots.

---

Release note:

Generation of release notes has been improved.

[Diaphteiros]

Let me know if you have proposals regarding the style of the generated release notes!